ARM: vexpress: switch to DT probe and multi-image build ...

This switches the VExpress support to use an internal DT, instead
of probing the peripherals from a board file. It also switches to
a multi-iamge build with both CA9 and CA15 variants of the VExpress
board being supported.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>