clk: add lpc18xx cgu clk driver

Add driver for NXP LPC18xx/43xx Clock Generation Unit (CGU). The CGU
contains several clock generators and output stages that route the
clocks either directly to peripherals or to a Clock Control Unit
(CCU).

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