arm: mx6: Add UART8 base address for i.MX6UL

Add the base address for the i.MX6UL so that this UART can be used.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>