esp8266: Move mp_hal_pin_open_drain from esp_mphal.c to machine_pin.c.

It belongs with the other pin config functions in machine_pin.c.  Also,
esp_mphal.c is put in iRAM so this change saves about 300 bytes of iRAM
(and mp_hal_pin_open_drain is not a time critical function so doesn't
need to be in iRAM).