clk: gxbb: expose CLKID_MMC_PCLK

The MMC_PCLK is needed for the SD/eMMC driver, expose to DT (and comment
out in clk driver)

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160707033837.20029-1-khilman@baylibre.com