xen/arm: Implement a virtual UART

xen/arm: Implement a virtual UART

This code is based on the previous vuart0 implementation. Unlike the latter,
it's intend to replace UART stolen by XEN to DOM0 via dtuart=... on its
command line.

It's useful when the kernel is compiled with early printk enabled or for a
single platform. Most of the time, the hardcoded code to handle the UART
will need 2 registers: status and data, the others registers can be
implemented as RAZ/WI.

This commit will also drop support of early printk (based on vexpress pl011)
in the guest.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Tim Deegan <tim@xen.org>