ARM: mstar: Add interrupt to pm_uart

Since we now have support for the interrupt controller pm_uart's
interrupt is routed through it make sense to wire up it's interrupt
in the device tree.

The interrupt is the same for all known chips so it goes in the
base dtsi.

Link: https://lore.kernel.org/r/20201002133418.2250277-4-daniel@0x0f.com


Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Olof Johansson <olof@lixom.net>