Add gem5 UART support to arm/qemu/virt_machdep.c

[bz]

Summary

While gem5 is not qemu, we treat it as "simulators" or "virtual environments".
Add the needed hardcoded gem5 attachments for the UART there, re-using all
the other bits.

In collaboration with: andrew
Sponsored by: DARPA/AFRL

[andrew]

Can you add a SOC_QEMU_VIRT and SOC_GEM5_VEXPRESS to options.arm & use this to check if we should include the platform support.

[bz]

In the efforts of making the GEM5 kernel config just being a VIRT config there seems to be no need to do the two different SOC options variants. As per discussion with andrew and I.