soc: dove: add legacy support to PMU driver

Add support for legacy non-DT Dove to the PMU driver, so that we can
transition the legacy support over.

[gregory.clement@free-electrons.com: removed pm_genpd_poweroff_unused]
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>