on an arch level, RISC-V defaults to FLATMEM. On PolarFire SoC, the
memory layout is almost always sparse, with a maximum of 1 GiB at
0x8000_0000 & a possible 16 GiB range at 0x10_0000_0000. The Icicle kit,
for example, has 2 GiB of DDR - so there's a big hole in the memory map
between the two gigs. Prior to v6.1-rc1, boot times from defconfig
builds were pretty bad on Icicle but enabling sparsemem would fix those
issues. As of v6.1-rc1, the Icicle kit no longer boots from defconfig
builds with the in-kernel devicetree. A change to the memory map
resulted in a futher "sparse-ification", producing a splat on boot:

	OF: fdt: Ignoring memory range 0x80000000 - 0x80200000
	Machine model: Microchip PolarFire-SoC Icicle Kit
	earlycon: ns16550a0 at MMIO32 0x0000000020100000 (options '115200n8')
	printk: bootconsole [ns16550a0] enabled
	printk: debug: skip boot console de-registration.
	efi: UEFI not found.
	Zone ranges:
	  DMA32    [mem 0x0000000080200000-0x00000000ffffffff]
	  Normal   [mem 0x0000000100000000-0x000000107fffffff]
	Movable zone start for each node
	Early memory node ranges
	  node   0: [mem 0x0000000080200000-0x00000000bfbfffff]
	  node   0: [mem 0x00000000bfc00000-0x00000000bfffffff]
	  node   0: [mem 0x0000001040000000-0x000000107fffffff]
	Initmem setup node 0 [mem 0x0000000080200000-0x000000107fffffff]
	Kernel panic - not syncing: Failed to allocate 1073741824 bytes for node 0 memory map
	CPU: 0 PID: 0 Comm: swapper Not tainted 5.19.0-dirty #1
	Hardware name: Microchip PolarFire-SoC Icicle Kit (DT)
	Call Trace:
	[<ffffffff800057f0>] show_stack+0x30/0x3c
	[<ffffffff807d5802>] dump_stack_lvl+0x4a/0x66
	[<ffffffff807d5836>] dump_stack+0x18/0x20
	[<ffffffff807d1ae8>] panic+0x124/0x2c6
	[<ffffffff80814064>] free_area_init_core+0x0/0x11e
	[<ffffffff80813720>] free_area_init_node+0xc2/0xf6
	[<ffffffff8081331e>] free_area_init+0x222/0x260
	[<ffffffff808064d6>] misc_mem_init+0x62/0x9a
	[<ffffffff80803cb2>] setup_arch+0xb0/0xea
	[<ffffffff8080039a>] start_kernel+0x88/0x4ee
	---[ end Kernel panic - not syncing: Failed to allocate 1073741824 bytes for node 0 memory map ]---

With the aim of keeping defconfig builds booting on icicle, enable
SPARSEMEM_MANUAL.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221021160028.4042304-1-conor@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>