dt-bindings: gpio: Add a binding header for the MSC313 GPIO driver

Header adds defines for the gpio number of each pad from the driver view.

The gpio block seems to have enough registers for 128 lines but what line
is mapped to a physical pin depends on the chip. The gpio block also seems
to contain some registers that are not related to gpio but needed somewhere
to go.

Because of the above the driver itself uses the index of a pin's offset in
an array of the possible offsets for a chip as the gpio number.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>