ARM: mach-shmobile: CPUIdle support

This patch adds a shared SH-Mobile ARM specific CPUIdle
implementation supporting WFI only at this point. It
serves as a common point for late registration of the
arch-specific CPUIdle code, and supports adding extra
sleep modes using the callback shmobile_cpuidle_setup()
together with shmobile_cpuidle_modes[].

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>