riscv: Add explicit include for cpu.h

Removing the include of cpu.h from of_device.h (included by
of_platform.h) causes an error in setup.c:

arch/riscv/kernel/setup.c:313:22: error: arithmetic on a pointer to an incomplete type 'typeof(struct cpu)' (aka 'struct cpu')

The of_platform.h header is not necessary either, so it can be dropped.

Link: https://lore.kernel.org/r/20230329-dt-cpu-header-cleanups-v1-8-581e2605fe47@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

arch/riscv/kernel/setup.c

@@ -8,6 +8,7 @@
  *  Nick Kossifidis <mick@ics.forth.gr>
  */
 
+#include <linux/cpu.h>
 #include <linux/init.h>
 #include <linux/mm.h>
 #include <linux/memblock.h>
@@ -15,7 +16,6 @@
 #include <linux/console.h>
 #include <linux/screen_info.h>
 #include <linux/of_fdt.h>
-#include <linux/of_platform.h>
 #include <linux/sched/task.h>
 #include <linux/smp.h>
 #include <linux/efi.h>