arm: mxs: Provide Kconfig option to to not use VDD5V as IMX28 PMU source

arm: mxs: Provide Kconfig option to to not use VDD5V as IMX28 PMU source

This option sets the current limit for 5V source to zero, so all
the PMU outputs are primarily powered from battery source (DCDC_BAT).

This option may be set on systems, where the 5V is NOT supposed to be
in any scenario powering the system - for example on systems where
DCDC_BAT is connected to fixed and regulated 4.2V source (so the
"battery" is not present).

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