ARM: vexpress: Device Tree updates

* Added extra regs for A15 VGIC
* Added A15 architected timer node
* Split A5 and A9 TWD nodes into two separate ones for timer
  and watchdog; interrupt definitions fixed on the way
* Fixed typo in A5 GIC compatible value

All the changes courtesy of Marc Zyngier.

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