clk: gxbb: add AmLogic GXBB clk controller driver

The gxbb clock controller is the primary clock generation unit for the
AmLogic GXBB SoC. It is clocked by a fixed 24MHz xtal, contains several
PLLs and the usual post-dividers, muxes, dividers and leaf gates that
are fed into various IP blocks in the SoC.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>