clk: gxbb: add MMC gate clocks, and expose for DT

Add the SD/eMMC gate clocks and expose them for use by DT.

While at it, also explose FCLK_DIV2 since this is one of the input
clocks to the mux internal to each of the SD/eMMC blocks.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>