MLK-16175-1 usb: ehci-mx6: Turn on the power domain of USB PHY

MLK-16175-1 usb: ehci-mx6: Turn on the power domain of USB PHY

Since there is no uclass for USB PHY. The device won't be setup for the USB PHY
node in DTB. And its associated power domain device won't be turned on neither
by DM framework.

This patch modifies the ehci-mx6 driver to enable the power domain device before
access the USB PHY. This is only for DM driver. For non-DM part, users still
need to power on the USB PHY in boards/SoC codes.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit 3c59c8ea95d401d06736c68c186aec80e214bee8)
(cherry picked from commit 79122202a4f748b5da654ed6e7ce6a4597eac747)