Update the documentation to support clock driver for the Amlogic G12B SoC.

G12B clock driver is very close, the main differences are :
- the clock tree is duplicated for the both clusters, and the
  SYS_PLL are swapped between the clusters
- G12B has additional clocks like for CSI an other components
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>