clk: meson: add regmap to the clock controllers

This change registers a regmap in meson8b, gxbb and axg controllers.
The clock are still accessing their registers directly through iomem.
Once all clocks handled by these controllers have been move to regmap,
the regmap register will be removed and replaced with a syscon request.

This is needed because other drivers, such as the HDMI driver, need to
access the HHI register region

Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>