iio: dac: add NXP LPC18xx DAC driver

Add base support for the 10-bit DAC peripheral found
on NXP LPC18xx/43xx SoCs.

This is a minimal driver that does not support DMA or
interrupts.

User manual with register description can be found on:
LPC18xx: www.nxp.com/documents/user_manual/UM10430.pdf
LPC43xx: www.nxp.com/documents/user_manual/UM10503.pdf

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>