remoteproc: Initialize rproc_class before use

The remoteproc_core and remoteproc drivers all initialize with module_init().
However remoteproc drivers need the rproc_class during their probe. If one of
the remoteproc drivers runs init and gets through probe before
remoteproc_init() runs, a NULL pointer access of rproc_class's `glue_dirs`
spinlock occurs.

> Unable to handle kernel NULL pointer dereference at virtual address 000000dc
> pgd = c0004000
> [000000dc] *pgd=00000000
> Internal error: Oops: 5 [#1] PREEMPT ARM
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper Tainted: G        W       4.14.106-rt56 #1
> Hardware name: Generic OMAP36xx (Flattened Device Tree)
> task: c6050000 task.stack: c604a000
> PC is at rt_spin_lock+0x40/0x6c
> LR is at rt_spin_lock+0x28/0x6c
> pc : [<c0523c90>]    lr : [<c0523c78>]    psr: 60000013
> sp : c604bdc0  ip : 00000000  fp : 00000000
> r10: 00000000  r9 : c61c7c10  r8 : c6269c20
> r7 : c0905888  r6 : c6269c20  r5 : 00000000  r4 : 000000d4
> r3 : 000000dc  r2 : c6050000  r1 : 00000002  r0 : 000000d4
> Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
...
> [<c0523c90>] (rt_spin_lock) from [<c03b65a4>] (get_device_parent+0x54/0x17c)
> [<c03b65a4>] (get_device_parent) from [<c03b6bec>] (device_add+0xe0/0x5b4)
> [<c03b6bec>] (device_add) from [<c042adf4>] (rproc_add+0x18/0xd8)
> [<c042adf4>] (rproc_add) from [<c01110e4>] (my_rproc_probe+0x158/0x204)
> [<c01110e4>] (my_rproc_probe) from [<c03bb6b8>] (platform_drv_probe+0x34/0x70)
> [<c03bb6b8>] (platform_drv_probe) from [<c03b9dd4>] (driver_probe_device+0x2c8/0x420)
> [<c03b9dd4>] (driver_probe_device) from [<c03ba02c>] (__driver_attach+0x100/0x11c)
> [<c03ba02c>] (__driver_attach) from [<c03b7d08>] (bus_for_each_dev+0x7c/0xc0)
> [<c03b7d08>] (bus_for_each_dev) from [<c03b910c>] (bus_add_driver+0x1cc/0x264)
> [<c03b910c>] (bus_add_driver) from [<c03ba714>] (driver_register+0x78/0xf8)
> [<c03ba714>] (driver_register) from [<c010181c>] (do_one_initcall+0x100/0x190)
> [<c010181c>] (do_one_initcall) from [<c0800de8>] (kernel_init_freeable+0x130/0x1d0)
> [<c0800de8>] (kernel_init_freeable) from [<c051eee8>] (kernel_init+0x8/0x114)
> [<c051eee8>] (kernel_init) from [<c01175b0>] (ret_from_fork+0x14/0x24)
> Code: e2843008 e3c2203f f5d3f000 e5922010 (e193cf9f)
> ---[ end trace 0000000000000002 ]---
Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Link: https://lore.kernel.org/r/20190530225223.136420-1-brandon.maier@rockwellcollins.comSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_core.c

@@ -2223,7 +2223,7 @@ static int __init remoteproc_init(void)
 
 	return 0;
 }
-module_init(remoteproc_init);
+subsys_initcall(remoteproc_init);
 
 static void __exit remoteproc_exit(void)
 {