bus: sunxi-rsb: Split out controller init/exit functions

This separates the resource acquisition from the hardware initialization
phase, so the hardware initialization can be repeated after system
suspend/resume. The same is done for the exit/remove function, except
that there is no resource deallocation phase due to the use of devres.

The requested RSB clock frequency is stored in `struct sunxi_rsb` so it
will be available when reinitializing the hardware.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>