xen: get GIC addresses from DT

xen: get GIC addresses from DT

Get the address of the GIC distributor, cpu, virtual and virtual cpu
interfaces registers from device tree.

Note: I couldn't completely get rid of GIC_BASE_ADDRESS, GIC_DR_OFFSET
and friends because we are using them from mode_switch.S, that is
executed before device tree has been parsed. But at least mode_switch.S
is known to contain vexpress specific code anyway.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>