pinctrl/meson: enable building as modules

Enable pinctrl drivers for 64-bit Amlogic SoCs to be built as modules.

The default is still built-in, this only adds the option of building
as modules.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20201026183025.31768-1-khilman@baylibre.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>