TF-A GICv3 driver: Introduce makefile

This patch moves all GICv3 driver files into new added
'gicv3.mk' makefile for the benefit of the generic driver
which can evolve in the future without affecting platforms.
The patch adds GICv3 driver configuration flags
'GICV3_IMPL', 'GICV3_IMPL_GIC600_MULTICHIP' and
'GICV3_OVERRIDE_DISTIF_PWR_OPS' described in
'GICv3 driver options' section of 'build-option.rst'
document.

NOTE: Platforms with GICv3 driver need to be modified to
include 'drivers/arm/gic/v3/gicv3.mk' in their makefiles.

Change-Id: If055f6770ff20f5dee5a3c99ae7ced7cdcac5c44
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 50cafcf..69e103d 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -667,6 +667,27 @@
    cluster platforms). If this option is enabled, then warm boot path
    enables D-caches immediately after enabling MMU. This option defaults to 0.
 
+GICv3 driver options
+--------------------
+
+GICv3 driver files are included using directive:
+
+``include drivers/arm/gic/v3/gicv3.mk``
+
+The driver can be configured with the following options set in the platform
+makefile:
+
+-  ``GICV3_IMPL``: Selects between GIC-500 and GIC-600 variants of GICv3.
+   This option can take values GIC500 and GIC600 with default set to GIC500.
+
+-  ``GICV3_IMPL_GIC600_MULTICHIP``: Selects GIC-600 variant with multichip
+   functionality. This option defaults to 0
+
+-  ``GICV3_OVERRIDE_DISTIF_PWR_OPS``: Allows override of default implementation
+   of ``arm_gicv3_distif_pre_save`` and ``arm_gicv3_distif_post_restore``
+   functions. This is required for FVP platform which need to simulate GIC save
+   and restore during SYSTEM_SUSPEND without powering down GIC. Default is 0.
+
 Debugging options
 -----------------