GICv3: GIC-600: Detect GIC-600 at runtime

The only difference between GIC-500 and GIC-600 relevant to TF-A is the
differing power management sequence.
A certain GIC implementation is detectable at runtime, for instance by
checking the IIDR register. Let's add that test before initiating the
GIC-600 specific sequence, so the code can be used on both GIC-600 and
GIC-500 chips alike, without deciding on a GIC chip at compile time.

This means that the GIC-500 "driver" is now redundant. To allow minimal
platform support, add a switch to disable GIC-600 support.

Change-Id: I17ea97d9fb05874772ebaa13e6678b4ba3415557
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index c863079..19b5653 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -711,8 +711,10 @@
 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_SUPPORT_GIC600``: Add support for the GIC-600 variants of GICv3.
+   Enabling this option will add runtime detection support for the
+   GIC-600, so is safe to select even for a GIC500 implementation.
+   This option defaults to 0.
 
 -  ``GICV3_IMPL_GIC600_MULTICHIP``: Selects GIC-600 variant with multichip
    functionality. This option defaults to 0