vexpress64: Enable VIRTIO_MMIO and RTC_PL031 in the base model

The Arm EBBR (Embedded Base Boot Requirements) require that the time
and basic networking EFI interfaces are available and working, so long
as the hardware has an RTC and network interface.

Arm FVPs typically have a memory-mapped PL031 RTC and a VIRTIO_NET
device defined in the device tree, so "imply" these in the Kconfig for
the FVP base model to simplify creating EBBR-compliant firmware.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>