xen/arm: Add MVEBU UART driver for Marvell Armada 3700 SoC

xen/arm: Add MVEBU UART driver for Marvell Armada 3700 SoC

This patch adds driver for UART controller found on Armada 3700 SoC.

There is no reference manuals available for 3700 SoC in public and it
is derived by looking at Linux driver[1].

[1]https://github.com/torvalds/linux/blob/master/drivers/tty/serial/mvebu-uart.c
commit-id: c685af1108d7c303f0b901413405d68eaeac4477

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>