1) dts/bindings/mmc/mmc.yaml: specifies common mmc.
2) dts/bindings/mmc/nxp,imx-usdhc.yaml: specifies
nxp usdhc module which inherits mmc.
3) dts/arm/nxp/nxp_rt.dtsi: usdhc support on RT chip.
4) boards/arm/mimxrt1050_evk/mimxrt1050_evk.dts:
usdhc slot support on mimxrt1050_evk board.
Signed-off-by: Jun Yang <jun.yang@nxp.com>