mmc: sdhci-pxav3: Get optional core clock

Besides the I/O clock, some PXAv3 SDHCI IP also requires a core clock to
be enabled. Add an optional core clock to the corresponding driver.

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