drivers: sensors: add veaa_x_3 proportional pressure regulator sensor

Add driver for Festo VEAA-X-3 series proportional pressure regulator.

The driver assumes that the maximum ADC value matches the maximum
output from the device, and that the maximum DAC value matches the
maximum input value for the device. External hardware is probably
required between the ADC/DAC and the device.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>