rk3399: disable the clock multiplier support when SoC init

The Clock Multiplier in rk3399 EMMC programmable clock generator
is broken, we can remove its support from SoC GRF register.

Without this patch, rk3399 emmc driver is not work after below patch
applied:
6dffdbc3 mmc: sdhci: Add the programmable clock mode support

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>