ARM: shmobile: sh73a0: Add CPU sleep suspend

Add the lighest possible sleep mode on Cortex-A9 cores: CPU sleep. It is
entered by a simple dsb and wfi instruction via cpu_do_idle(). As just
clocks are stopped there is no need to save or restore any state of the
system.

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>