clk: meson: migrate mplls clocks to clk_regmap

Rework meson mpll driver to use clk_regmap and move meson8b, gxbb
and axg clocks using meson_clk_mpll to clk_regmap

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>