clk: add lpc18xx ccu clk driver

Add driver for NXP LPC18xx/43xx Clock Control Unit (CCU). The CCU
provides fine grained gating of most clocks present in the SoC.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>