arm64: dts: imx8qm-mek: fix gpio number for reg_usdhc2_vmmc

The gpio in "reg_usdhc2_vmmc" should be 7 instead of 19.

Cc: stable@vger.kernel.org
Fixes: 307fd14d

 ("arm64: dts: imx: add imx8qm mek support")
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>