ARM: shmobile: Use domain names when adding subdomains to power domains

Make the power management code under arch/arm/mach-shmobile/
use pm_genpd_add_subdomain_names() for adding subdomains to power
domains, which makes it possible to drop
rmobile_pm_add_subdomain() and will allow us to carry out those
operations for domain objects stored in tables in a straightforward
way.

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