irqchip/gic-v3-its: Rename MAPVI to MAPTI

Back in the days when the GICv3/v4 architecture was drafted,
the command to an event to an LPI number was called MAPVI.
Later on, and to avoid confusion with the GICv4 command VMAPI,
it was renamed MAPTI. We've carried the old name for a long
time, but it gets in the way of people reading the code in
the light of the public architecture specification.

Just repaint all the references and kill the old definition.

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