irqchip/gic-v3-its: Set implementation defined bit to enable VLPIs

A long time ago, GITS_CTLR[1] used to be called GITC_CTLR.EnableVLPI.
It has been subsequently deprecated and is now an "Implementation
Defined" bit that may ot may not be set for GICv4. Brilliant.

And the current crop of the FastModel requires that bit for VLPIs
to be enabled. Oh well... Let's set it and find out what breaks.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>