BCM270x: power: Change initcall level to subsys

Load ordering of modules are determined by the initcall used.
If it's the same initcall level, makefile ordering decides.
Now that the mailbox driver is being moved, it's no longer
placed before the power driver by the linker.
So use a later initcall level to let the mailbox driver
load first.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>