configs: Raise 8250 UART limit to 5 on BCM2711

8250 UARTs are available as PCIe devices, and there is already a config
setting enabling the driver support, but the driver also has a hard
limit on the number of instances - currently set to 1. Raise that
limit to 5.

See: https://github.com/raspberrypi/linux/issues/5590
     https://forums.raspberrypi.com/viewtopic.php?t=355689



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