dt-bindings: mmc: imx-esdhc: Add imx8ulp compatible string

The USDHC on i.MX8ULP is derived from i.MX8MM, it uses two
compatible strings, so update the compatible string for i.MX8ULP.

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20211112082930.3809351-3-peng.fan@oss.nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>