arm: Allow the user to specify the GIC version

arm: Allow the user to specify the GIC version

A platform may have a GIC compatible with previous version of the
device.

This is allow to virtualize an unmodified OS on new hardware if the GIC
is compatible with older version.

When a guest is created, the vGIC will emulate same version as the
hardware. Although, the user can specify in the configuration file the
preferred version (currently only GICv2 and GICv3 are supported).

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>