dt-bindings: clock: add description of LPC32xx clock controller

NXP LPC32xx SoC has a clocking and power control unit (CPC) as a part
of system control block (SCB). CPC is supplied by two external
oscillators and it manages core and most of peripheral
clocks, the change adds description of DT bindings for clock
controller found on LPC32xx SoC series.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>