Instead of introducing new functions each time a new power supply
situation is to be added, this patch adds parameters to mx28_power_init.

Right now there are three parameters:

- has_battery - true when this board has a battery.
- use_battery_input - true when this board is supplied from the
  battery input, but has a DC source instead of a real battery
- use_5v_input - true when this board can use the 5V input

The third one is introduced with this patch and allow to boot a board from 5v
(USB) source only. The main necessary change this needs is that the DC-DC
converter must always be sourced from DCDC_4P2 (DROPOUT_CTRL field of
HW_POWER_DCDC4P2)

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>