ARM: dts: imx7-mba7: Use 'no-mmc' property

'no-emmc' is not a valid property.

The original intention was to use the 'no-mmc' property.

Change it accordingly to fix the following dt-schema warning:

imx7s-mba7.dtb: mmc@30b40000: Unevaluated properties are not allowed ('no-emmc' was unexpected)

Fixes: d430a7e0 ("ARM: dts: imx7-mba7: restrict usdhc interface modes")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>