dt-bindings: pinctrl: meson: update register descriptions

like pull-enable, pull should be optional has this region is available on
every controllers. Also, the g12a feature a new region "ds" for the
drive-strength

All this region thing is one big mess. I suspect that there is only one
big GPIO region with holes in it. All registers between the current
regions reads '0' so it is probably just spare space to handle more pins.

Since we need to continue to handle the existing controllers, switching to
one single region now would not simplify things. However, if more
organisation layouts and features keep on being added, we may have to look
at this again

Fixes: 3cd3c83f ("pinctrl: Add compatibles for Amlogic Meson G12A pin controllers")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>