openrisc: Include cpu.h and switch_to.h for prototypes

When compiling with W=1 enabling -Wmissing-prototypes the compiler
warns:

  arch/openrisc/kernel/process.c:100:6: error: no previous prototype for 'arch_cpu_idle' [-Werror=missing-prototypes]
  arch/openrisc/kernel/process.c:240:21: error: no previous prototype for '__switch_to' [-Werror=missing-prototypes]

Fix these by adding the approrpiate header files to process.c which
brings in the prototype definitions.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/linux-kernel/20230810141947.1236730-17-arnd@kernel.org/


Signed-off-by: Stafford Horne <shorne@gmail.com>