AP_HAL: define RC_12

we have 12 channels on PX4FMU+PX4IO
HAL_PX4: support 12 output channels with PX4IO

first 8 on IO board, next 4 on FMU outputs
HAL_PX4: setup for 12 output channels

moved telemetry port to /dev/ttyS2 to free PWM outputs on FMU
RC_Channel: support 8 auxillary RC channels

needed for full 12 channel support on PX4
Copter: support all 12 channels on PX4

last 4 channels are on the FMU pins
Plane: if airspeed enabled, require 5m/s to unsuppress throttle
Plane: support 12 output channels on PX4