mmc: dt-bindings: add "bus-clk" for MT2712

On MT2712 MSDC0/3, HCLK/bus-clk need gate/ungate together,
or will hang when access MSDC register.
Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mtk-sd.txt

@@ -22,6 +22,7 @@ Required properties:
 	"source" - source clock (required)
 	"hclk" - HCLK which used for host (required)
 	"source_cg" - independent source clock gate (required for MT2712)
+	"bus_clk" - bus clock used for internal register access (required for MT2712 MSDC0/3)
 - pinctrl-names: should be "default", "state_uhs"
 - pinctrl-0: should contain default/high speed pin ctrl
 - pinctrl-1: should contain uhs mode pin ctrl