gpio: qpnp-pin: Support for selecting DTEST in DIG_IN configuration

The GPIO/MPP can be configured to be routed to the DTEST
line in DIG_IN configuration. Add a parameter 'dtest-sel'
to select and enable the DTEST configuration.

Change-Id: Ie051745da0e76866ad59105e4bf54411f03681f9
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>