clk: meson: add regmap clocks

Meson clock controllers need to move the classical iomem registers to
regmap. This is triggered because the HHI controllers found on the GXBB
and GXL host more than just clocks. To properly handle this, we would
like to migrate HHI to syscon. Also GXBB AO clock controller already use
regmap, AXG AO and Audio clock controllers will as well.

The purpose of this change is to provide a common structure to these
meson controllers (and possibly others) for regmap based clocks.

This change provides the basic gate, mux and divider, based on the
helpers provided by the related generic clocks

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