xen/arm: gic-v3: Clean-up the GIC*_PIDR2_* definitions

xen/arm: gic-v3: Clean-up the GIC*_PIDR2_* definitions

GICR_PIDR2 and GICD_PIDR2 use the same register layout. Rather than
define twice, one of which is an alias to the other, introduce GIC_PIDR2_*
defines.

Also:
    * Use the same prefix for the mask and the value
    * Integrate the shift in the value to avoid shifting in the code
    * Use GICv* to match the value name in the spec
    * Move them in a proper place

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>