staging: olpc_dcon: Convert all uses of old GPIO API to new descriptor API

This commit eliminate all uses of legacy integer base GPIO API in
olpc_dcon_xo_1_5.c and replace them with new descriptor GPIO API like
those in olpc_dcon_xo_1.c.

Also pull some common code with olpc_dcon_xo_1.c to olpc_dcon.h for code
sharing.

Signed-off-by: Jerry Lin <wahahab11@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>