ARM: dts: imx6ull: add UART8 support

In i.MX 6ULL UART8 is part of the AIPS-3 memory map instead of
AIPS-1. Clocks and interrupts remain the same.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>