ARM: shmobile: r8a7740: 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>
[ horms@verge.net.au: Added missing includes ]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>