irqchip/gic-v3-its: Drop deprecated GITS_BASER_TYPE_CPU

During the development of the GICv3/v4 architecture, it was
envisaged to have a CPU table, though the use for it was
never completely clear (the collection table serves that role
pretty well). It ended being dropped before the specification
was published, though it lived on in the driver.

In order to avoid people scratching their head too much, let's do
the same in the kernel.

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