arm64: dts: marvell: add UART muxing on Armada 7K/8K

This commit adds the relevant details in the Armada 7K/8K Device Tree
to properly mux the UART used for the serial console. Since there is
basically only one possible muxing for the UART0 on the AP, the muxing
configuration is described in armada-ap806.dtsi, and selected from the
individual boards (other boards could be using a different UART).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>