riscv: Add explicit include for cpu.h

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.org
Signed-off-by: Rob Herring <robh@kernel.org>