sh: cpuidle for SuperH Mobile using hwblk

This patch adds cpuidle support for SuperH Mobile.

The sleep mode selected by cpuidle is compared with
the mode selected by the hwblk sleep code and the
best allowed mode is entered.

At this point "Sleep mode" and "Sleep mode + SF" are
supported. This code can easily be extended to support
"Software suspend mode", but the assembly code must
first be updated to avoid loosing interrupts.

Also, update the code to only copy the assembly snippet
into internal memory once at bootup.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>