clk: meson: add dual divider clock driver

Add the dual divider driver. This special divider make a weighted
average between 2 dividers to reach fractional divider values.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181221160239.26265-4-jbrunet@baylibre.com