mmc: sdhci-pxav3: Move I/O clock to private data

As we are using references to the I/O clock throughout the driver,
move it to the private data. Also, in preparation for core clock,
rename it to clk_io.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>