This patch fixes the offset of GICD_IROUTER register defined in gicv3.h.
Although the GICv3 documention mentions that the offset for this register
is 0x6100-0x7FD8, the offset calculation for an interrupt id `n` is :

   0x6000 + 8n, where n >= 32

This requires the offset for GICD_IROUTER to be defined as 0x6000.

Fixes ARM-software/tf-issues#410

Change-Id: If9e91e30d946afe7f1f60fea4f065c7567093fa8