USB: serial: cp210x: add GPIO support for CP2104

The CP2104 chips feature 4 controllable GPIO pins, which are similar to
the ones on CP2102N chip (output-only when push-pull, output or
simulated input mode when open-drain).

Add support for the GPIO pins for cp210x driver. The pin get/set routine
is shared with CP2102N, but the pinconf initialization code is not
shared because the acquisition of GPIO configuration in OTP ROM is
similar to CP2105, not CP2102N.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Johan Hovold <johan@kernel.org>