dt-bindings: mmc: Add missing description for clk_in/out_sd1

dt-bindings: mmc: Add missing description for clk_in/out_sd1

The commit a8fdb80f4d47 ("arm64: zynqmp: Add ZynqMP SDHCI compatible
string") added clock-output-names for both SDHCIs before DT binding yaml
conversion. But only clk_in/out_sd0 clock names have been covered by
DT binding which ends up with dt yaml checking warnings as:
From schema: .../Documentation/devicetree/bindings/mmc/arasan,sdhci.yaml
... mmc@ff170000: clock-output-names:0: 'clk_out_sd0' was expected
... mmc@ff170000: clock-output-names:1: 'clk_in_sd0' was expected

Fixes: 16ecd8f33c6e ("dt-bindings: mmc: convert arasan sdhci bindings to yaml")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Manish Narani <manish.narani@xilinx.com>
Link: https://lore.kernel.org/r/aef586778921c93377ec2f31c86e151b6e93f6c7.1598257520.git.michal.simek@xilinx.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>