imx: imx8mq: handle ESDHC in mxc_get_clock

imx: imx8mq: handle ESDHC in mxc_get_clock

fsl_esdhc_imx driver will call "mxc_get_clock(MXC_ESDHC_CLK +
dev->seq)", however mxc_get_clock wrongly handle MXC_ESDHC_CLK
as root clk and cause sd card could not be detected in U-Boot proper,
as below:
"Loading Environment from MMC... unable to select a mode"

Handle MXC_ESDHC_CLK in mxc_get_clock to fix the issue.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Fabio Estevam <festevam@gmail.com>