pinctrl: meson: rework pinmux ops

This change prepare the introduction of new meson SoC. This new SoC will
share the same gpio/pinconf registers but the pinmux part will be
different. While the format of the data associated with each pinmux group
will change, the way to handle pinmuxing will be similar.

To deal with this new situation, the meson_pmx_struture is kept but the
data associated to it is now generic. This allows to reuse the basic
functions which would otherwise be copy/pasted in each pinmux driver
(such as getting the name a count of groups and functions) Only the
functions actually using this specific data is taken out of the common
code and is handling the SoC pinmuxing

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>