ARM: vexpress: Add Cortex-R Series UART, selectable via DEBUG_LL

The Cortex-R series processors on Versatile Express have a different memory
map to the RS1 and CA9X4 tiles. Most of the platform difference can be
expressed in device-trees, but the UART definitions for LL_DEBUG cannot.

This patch defines the UART location for R-Series processors on
versatile-express, allowing low-level debug and output from the decompressor.
These definitions are selectable via Kconfig

Signed-off-by: Jonathan Austin <jonathan.austin@arm.com>
CC: Pawel Moll <pawel.moll@arm.com>