arm: mxs: Provide Kconfig option to not enable 4P2 regulator in IMX28 PMU

arm: mxs: Provide Kconfig option to not enable 4P2 regulator in IMX28 PMU

The IMX28 PMU (Power Management Unit) has a dedicated Linear Regulator
to produce (by default) 4.2V output - available outside the chip as
VDD_4P2.

When system is supposed to not use VDD5V as a main power source - instead
the DCDC_BATT is used; it is safe to disable this regulator.

As the in-PMU DCDC switching regulator (from which DCDC_VDDA, DCDC_VDDIO
and DCDC_VDDD are generated) can be driven from DCDC_BATT or output
of this 4P2 regulator - by disabling the latter the use of the DCDC_BATT
is forced.

To be more specific - according to NXP's AN4199 the DCDC_BATT source is
preferred (over VDD5V), as more efficient and stable source for
industrial applications.

Signed-off-by: Lukasz Majewski <lukma@denx.de>