gpio: Add GPIO voltage overrides

Add GPIO_flag values for GPIO DTS creation that
allows for pins that do not use the default
voltage.  IT8xxx2 supports setting voltage
levels for each pin. The default for this SoC
is 3P3 but they can be set individually to
1P8.

This will be used, for example:

    i2c_c_scl {
            gpios = <&gpioc 1 (GPIO_INPUT | GPIO_VOLTAGE_1P8)>;
            enum-name = "GPIO_I2C_C_SCL";
            label = "I2C_C_SCL";
    };

Signed-off-by: Denis Brockus <dbrockus@google.com>