arm64: dts: marvell: armada-3720-espressobin: fill UART nodes

Fill ESPRESSObin uart0 node with pinctrl information like in the
Armada-3720-DB device tree (which uses the same node).

Also explain how to enable the second UART port available on the
headers. This second port is not enabled by default because both
headers are dedicated to expose general purpose pins and remapping
some of them to use the second UART would break existing users.

Suggested-by: László ÁSHIN <laszlo@ashin.hu>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>