irqchip/gic-v3-its: Refactor command encoding

The way we encode the various ITS command fields is both tedious
and error prone. Let's introduce a helper function that performs
the encoding, and convert the existing encoders to use that
helper. It also has the advantage of expressing the encoding in
a way that matches the architecture specification.

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