mfd: mxs-lradc: Add support for mxs-lradc

Add core files for low resolution analog-to-digital converter (mxs-lradc)
MFD driver.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>