usb: ehci-mx6: Fix aarch64 build warnings

Fix cast from pointer to integer of different size by casting the
pointer to uintptr_t instead of uint32_t, the former has correct
size on both 32bit and 64bit architectures.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>