clk: mvebu: adjust clock handling for the CP110 system controller

This commit:

 - makes the GOP_DP (bit 9) gatable clock a child clock of the
   SD_MMC_GOP (bit 18) clock, as it should have been. The clock for bit
   18 was just named SD_MMC, but since it also covers the GOP block, it
   is renamed SD_MMC_GOP.

 - makes the MG (bit 5) gatable clock a child clock of the MG_CORE
   clock (bit 6)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>