mmc: sunxi: Add more debug informations

Add a bit more debug messages that can be helpful when debugging the clock
setup.

Also fill the actual_clock field in struct mmc_host to report properly the
current frequency in debugfs.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>