ARM: vexpress: Remove motherboard dependencies in the DTS files

The way the VE motherboard Device Trees were constructed
enforced naming and structure of daughterboard files. This
patch makes it possible to simply include the motherboard
description anywhere in the main Device Tree and retires
the "arm,v2m-timer" alias - any of the motherboard SP804
timers will be used instead.

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