ARM: Dummy Virtual Machine platform support

Add support for the smallest, dumbest possible platform, to be
used as a guest for KVM or other hypervisors.

It only mandates a GIC and architected timers. Fits nicely with
a multiplatform zImage. Uses very little silicon area.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[will: fixed up conflicts with GIC move to drivers/irqchip/]
Signed-off-by: Will Deacon <will.deacon@arm.com>