PM / shmobile: Add support for the sh7372 A4S power domain / sleep mode

The sh7372 contains a power domain named A4S which in turn
contains power domains for both I/O Devices and CPU cores.

At this point only System wide Suspend-to-RAM is supported,
but the the hardware can also support CPUIdle. With more
efforts in the future CPUIdle can work with bot A4S and A3SM.

Tested on the sh7372 Mackerel board.

[rjw: Rebased on top of the current linux-pm tree.]

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>