mmc: fsl_esdhc_imx: Add i.MX8MP compatible string

Add compatible string for i.MX8MP, which permits i.MX8MP to use
HS400ES mode, just like all the other i.MX8M.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Haibo Chen <haibo.chen@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>