arm64: dts: marvell: armada-37xx: add UART clock

Add the missing clock property to armada-3700 UART node.

This clock will be used to derive the prescaler value to comply with
the requested baudrate.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>