MLK-15013-1 ehci-mx6: Add i.MX8 OTG controller support

The i.MX8 has two USB controllers: USBOH and USB3. The USBOH reuses
previous i.MX6/7. It has same PHY IP as i.MX7ULP but NC registers
are same as i.MX7D. So add its support in ehci-mx6 driver.

Also the driver is updated to remove build warning for 64 bits CPU.

Signed-off-by: Ye Li <ye.li@nxp.com>