ARM: 6771/1: vexpress: add support for multiple core tiles

The current Versatile Express BSP defines the MACHINE_START macro
in the core tile code.

This patch moves this into the generic board code and introduces a
method for determining the current tile at runtime, allowing the
Kernel to have support for multiple tiles compiled in. Tile-specific
functions are executed via a descriptor struct containing the correct
implementations for the current tile.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>