doc: dt: arm,pl172: add description of PL175 and PL176 controllers

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Joachim Eastwood <manabian@gmail.com>