arm64: dts: fvp/juno: Fix serial node names

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.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>