imx8qxp_mek: Start with USDHC2 unpowered

imx8qxp_mek: Start with USDHC2 unpowered

After triggering a "reboot" command in Linux the following
hang in SPL is observed:

U-Boot SPL 2020.04 (Apr 15 2020 - 10:49:29 -0300)
Normal Boot
WDT:   Not found!
Trying to boot from MMC2_2
spl: mmc init failed with error: -70
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

This error happens because the CMD_ERR bit is set in the IRQ status
register after booting from Linux.

To ensure a fresh start, force the VMMC supply to get unpowered first.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>