dt-bindings: Convert gpio-mmio to yaml

This is a generic binding for simple MMIO GPIO controllers. Although we
have a single driver for these controllers, they were previously spread
over several files. Consolidate them. The register descriptions are
adapted from the comments in the source. There is no set order for the
registers, and some registers may be omitted. Because of this, reg-names
is mandatory, and no order is specified.

Rename brcm,bcm6345-gpio to brcm,bcm63xx-gpio to reflect that bcm6345
has moved.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>