This patch adds the IIDR value for GIC600-AE to the gicv3_is_gic600()
helper function. This helps platforms supporting this version of the
GIC600 interrupt controller to function with the generic GIC driver.

Verified with tftf-validation test suite

******************************* Summary *******************************
> Test suite 'Framework Validation'
                                                                Passed
> Test suite 'Timer framework Validation'
                                                                Passed
=================================
Tests Skipped : 0
Tests Passed  : 6
Tests Failed  : 0
Tests Crashed : 0
Total tests   : 6
=================================
NOTICE:  Exiting tests.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I518ae7b56f7f372e374e453287d76ca370fc3574