Revert "clk: gxbb: expose CLKID_MMC_PCLK"

This reverts commit e16fb2e6

.

Updated documentation from the chip vendor reveals that this clock is
not required for correct operation of the MMC controller. As such, do
not expose it to DT.

Signed-off-by: Michael Turquette <mturquette@baylibre.com>