ARM: shmobile: Allow use of boot code for non-SMP case

Allow build of platsmp.c and headsmp.S even though
SMP is disabled in the kernel configuration. With
this in place it is possible to share the reset
vector setup code with power management code that
needs to be built even though SMP is disabled.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>