powerpc/rcpm: Fix build break when SMP=n

Add an include of asm/smp.h to fix a build break when SMP=n:

  arch/powerpc/sysdev/fsl_rcpm.c:32:2: error: implicit declaration of
  function 'get_hard_smp_processor_id'

Fixes: d17799f9 ("powerpc/rcpm: add RCPM driver")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>