gpio: vx855: use the new open drain callback

The vx855 driver clearly states it has three groups of lines:
GPI, GPO and GPIO. The GPO are assumedly push-pull. The GPIO
are implicit open drain, but if the GPIO subsystem ask for them
to be explicitly open drain (i.e. set the flag on a machine table
that we want open drain) it will currently misbehave: it will
switch the GPIOs to input mode (emulate open drain). Instead:
indicate in the .set_single_ended() callback that we support
open drain and open drain only.

Cc: Daniel Drake <drake@endlessm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>