The UARTs for all Arm Ltd. boards were using "uart" as their node name
stub.

Replace that with the required "serial" string, to comply with the PL011
DT binding.

Link: https://lore.kernel.org/r/20200513103016.130417-14-andre.przywara@arm.comSigned-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>