ARM: mvebu: add UART labels to Armada 375

This commit adds the standard uart0 and uart1 DT labels to the Device
Tree description of the Marvell Armada 375 SoC.

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