ARM / shmobile: Make A3RV be a subdomain of A4LC on SH7372

Instead of coding the undocumented dependencies between power domains
A3RV and A4LC on SH7372 directly into the low-level power up/down
routines, make A3RV be a subdomain of A4LC, which will cause the
same dependecies to hold.

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