ARM: dts: bcm2711: Add extra serial aliases

With UART numbering from DT aliases re-enabled, add aliases for the
additional BCM2711 UARTs. Also use the opportunity to remove some
DTS duplication.

See: https://forums.raspberrypi.com/viewtopic.php?t=347868



Signed-off-by: Phil Elwell <phil@raspberrypi.com>