arm64: dts: imx8mm-nitrogen-r2: rework USDHC1

Add VMMC and VQMMC description for USDHC1 (eMMC).
There are comming directly from the alimentation
stage, so add the vref_3V3 fixed regulator.

Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>