clk: add lpc18xx creg clk driver

The CREG block on lpc18xx contains configuration register
for two low power clocks. Support enabling of these two
clocks with a clk driver that access CREG trough the
syscon regmap interface.

These clocks are needed to support peripherals like the
internal RTC on lpc18xx.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>