iio: adc: add NXP LPC18xx ADC driver

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

This is a minimal driver that does not support burst mode,
interrupts, DMA or hardware triggers.

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>