arm: dts: imx7s-warp: Create alias for mmc0 to &usdhc3

arm: dts: imx7s-warp: Create alias for mmc0 to &usdhc3

This patch sets up an alias for mmc0 to usdhc3.

Before the DM conversion only usdhc3 was enabled and therefore it appeared
as MMC 0 to u-boot. After enabling MMC DM though usdhc3 defaults to MMC 2,
which left unattended would drive changes to existing warp7 bootscripts and
environment variables that rely on mmc 0.

Setup the alias of mmc0 and usdhc3 so that existing warp7 boot code will
work unmodified.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>