Support PWM and H bridge GPIO control for the Robotics cape.

- Optimize MSS GPIO_2 usage.
- GPIOS build option now uses GPIOs for all cape pins except I2C on
  P9_PIN19 and P9_PIN20 which are hard-wired to MSS I2C.
- Move user LEDs control to MSS GPIO_2.

Let I/O pads propagate to the top level of the design and rename as
required.