dt: binding: reword PowerPC 8xxx GPIO documentation

re-format and re-word the device tree binding documentation for MPC8xxx
and compatibles, reference the common document for interrupt controllers
and remove outdated duplicate SoC specific information

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <Pawel.Moll@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: <devicetree@vger.kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>