mx6: soc: Update the comments of set_ldo_voltage()

mx6: soc: Update the comments of set_ldo_voltage()

Commit 3d622b78 (mx6: soc: Introduce set_ldo_voltage()) introduces
set_ldo_voltage() function that can be used to set the voltages
of any of the three LDO regulators controlled by the PMU_REG_CORE register.

Prior to this commit there was a single set_vddsoc() which only configured the
VDDSOC regulator.

Update the comments to align with the new set_ldo_voltage() implementation.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>