ARM: config: Enable MMC_CLKGATE flag on SDM845

Enabling mmc clock gating on SDM845. With this feature
eanbled, mmc clock shall be gated when there is no clock
request on the mmc host.

Change-Id: I011a9b680363bc68655c30b062753b5272d1fae0
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>