MMC nodes has to be named mmc@ to comply with dt-bindings.

Fix warnings as:
arch/arm/boot/dts/qcom-msm8660-surf.dt.yaml: sdcc@12400000: $nodename:0: 'sdcc@12400000' does not match '^mmc(@.*)?$'
	From schema: Documentation/devicetree/bindings/mmc/arm,pl18x.yaml
Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211020201440.229196-1-david@ixit.cz