mmc: sdhci-pxav2: add optional core clock

mmc: sdhci-pxav2: add optional core clock

Add ability to have an optional core clock just like the pxav3 driver.
The PXA168 needs this because its SDHC controllers have separate core
and io clocks that both need to be enabled. This also correctly matches
the documented devicetree bindings for this driver.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Doug Brown <doug@schmorgal.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20230116194401.20372-6-doug@schmorgal.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>