pinctrl: meson: separate soc drivers

When meson pinctrl is enabled, all meson platforms pinctrl drivers are
built in the kernel, with a significant amount of data.

This leads to situation where pinctrl drivers targeting an architecture
are also compiled and shipped on another one (ex: meson8 - ARM - compiled
and shipped on ARM64 builds). This is a waste of memory we can easily
avoid.

This change makes 4 pinctrl drivers (1 per SoC) out the original single
driver, allowing to compile and ship only the ones required.

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>