mfd: Improve mc13xxx dt binding document

It improves mc13xxx dt binding document on how the regulator name
is being used for binding a mc13892 regulator device.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>