ARM: dts: vfxxx: Pass 'mmc' as the esdhc node names

Pass 'mmc' as the esdhc node names to fix the following schema
warnings:

vf610-cosmic.dtb: esdhc@400b1000: $nodename:0: 'esdhc@400b1000' does not match '^mmc(@.*)?$'

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>