ARM: vexpress: Make the debug UART detection more specific

Base the UART detection heuristic on architecturally defined
MIDR register instead of implementation dependent CBAR. The
only tile using the original memory map is V2P-CA9 with Cortex
A9 r0p1, which MIDR contains value 0x410fc091.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>