dt-bindings: mfd: Move mxs-lradc binding document from IIO to MFD

The bindings, which are now used in MFD, need also to be
documented in the MFD binding document.

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>