da850-evm: extract defines for SEL{A,B,C} pins in UI expander

The setup and teardown methods of the UI expander reference the SEL_{A,B,C}
pins by 'magic number' in each function. This uses the common enum for their offsets
in the expander setup and teardown functions.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Reviewed-by: Chris Cordahi <christophercordahi@nanometrics.ca>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
CC: Victor Rodriguez <vm.rod25@gmail.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>