MFC r333255, r333260

r333255:
Make reading imx6 gpio pins work correctly whether the pin is in open-drain
mode or not. An earlier attempt to make this work was done in r320456, by
always reading the pad status register (PSR) instead of the data register.
But it turns out the values in PSR only reflect the electrical level of an
output pin if the pad is configured with the SION (Set Input On) bit in the
pinmux config, and most output gpio pads are not configured that way.

So now a gpio read is done by returning the value from the data register,
which works right whether the pin is configured for input or output, unless
the pin has been set for OPENDRAIN mode, in which case the PSR is read
instead. For this to work, the pin must also be configured with SION turned
on in the fdt pinmux data, which is a reasonable thing to require for the
unusual case of reading an open-drain output pin.

r333260:
Properly support the GPIO_PIN_PRESET_{LOW,HIGH} options when configuring
a gpio pin. If neither of the options is specified, pre-set the pin's
output value to the pin's current input value, to achieve glitch-free
transitions to output mode on pins that are pulled up or down at reset
or via fdt pinctrl data.