drivers: mfd: axp192: add possibility to disable N_VBUSEN functionality

By default N_VBUSEN signal is used by the host to define if VBUS
signal should be used for power supply (VBUS-IPSOUT).

This feature enables the possbibility to change this behaviour.
When N_VBUSEN detection is disabled, axp192 will automatically
decide to use VBUS (REG10H[7] = 1).
Please refer to datasheet for details.

Signed-off-by: Martin Kiepfer <mrmarteng@teleschirm.org>