drivers: gpio: axp192: Add support to control EXTEN pin

AXP192 features an EXTEN pin that is on output only.

This commit appends control of EXTEN pin to gpio functionality
of AXP192.
Port-Mapping is as follows:
- [0..4]: GPIO0..4
- [5]: EXTEN

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