GPIO, RCOutput, RCInput(skeleton) drivers for Navio

Patchset includes:
Separation of BBB and RPI GPIO drivers
GPIO driver for Raspberry Pi based on direct register control
RCOutput working over I2C PWM chip PCA9685
Changed initialization order to allow I2C PWM chip to be initialized after I2C initialization
Separation of PRU and Navio's RCInput (Navio RCInput to be added later)