ARM: vexpress: Add Device Tree for V2P-CA15_CA7 core tile

This patch adds Device Tree file for the CoreTile Express
A15x2 A7x3 (V2P-CA15_CA7).

Note that the A7 cpu nodes are commented out, as the
big.LITTLE-relevant patches are not upstreamed yet. Till
this time one can use the board with two A15 cores only,
keeping the A7s in reset by adding the following setting
to the board.txt file in Versatile Express configuration
tree:

SCC: 0x018 0x00001FFF

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