ARM: vexpress: Get rid of MMIO_P2V

This patch gets rid of the MMIO_P2V and __MMIO_P2V macros,
defining constant virtual base for motherboard and tile
peripherals instead.

Additionally, in preparation for the new motherboard memory
map, the motherboard peripherals are using base pointers
calculated in runtime, instead of compile-time calculated
values.

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