mmc: sunxi: Move resources management to separate functions

We've had all our resources management, and especially the clocks and reset
sequence, done directly as part of the probe.

As we want to implement runtime_pm, we'll obviously want to have that
moved outside of the probe so that we can call do it in our runtime suspend
and resume hooks without too much duplication.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>