pinctrl: meson: get rid of unneeded domain structures

The driver originally supported more domains (register ranges) per
pinctrl device, but since commit 9dab1868 ("pinctrl: amlogic: Make
driver independent from two-domain configuration") each device gets
assigned a single domain and we instantiate multiple pinctrl devices
in the DT.

Therefore, now the 'meson_domain' and 'meson_domain_data' structures
don't have any reason to exist and can be removed to make the model
simpler to understand. This commit doesn't change behavior.

Tested on a Odroid-C2.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>