arm: scb: Move SCB asm defines into cpu_idle.S

As cpu_idle.S is the only bit of code that is using the SCB asm defines,
so to allow us to remove scb.h in the future lets move the defines that
are used just into cpu_idle.S

Jira: ZEP-1568

Change-Id: I3c3a6f145ec4c1a43f076d079d5fe1694c255b78
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>