ARM: shmobile: Drop r8a7779_add_device_to_domain()

If the r8a7779's PM domains are given names, this SoC and its boards
will be able to use rmobile_add_device_to_domain() for adding devices
to those domains and r8a7779_add_device_to_domain(), which is not
used anywhere at the moment anyway, may be dopped.

Accordingly, give names to the r8a7779's PM domains and drop
r8a7779_add_device_to_domain().

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