cc2538: Clean up port and pin definitions

Homogenize port and pin definitions naming:
 - PERIPHERAL_FUNCTION_PORT for the port ID,
 - PERIPHERAL_FUNCTION_PIN for the pin ID,
 - PERIPHERAL_FUNCTION_PORT_BASE for the port base,
 - PERIPHERAL_FUNCTION_PIN_MASK for the pin mask.

Define only PERIPHERAL_FUNCTION_PORT and PERIPHERAL_FUNCTION_PIN in board.h, and
deduce PERIPHERAL_FUNCTION_PORT_BASE and PERIPHERAL_FUNCTION_PIN_MASK in the
driver from the former definitions.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: usb: Make the GPIO driving the pull-up optional

The data sheet recommends that the USB pull-up resistor be driven by a GPIO so
that it can be controlled by software, but this is not mandatory. Hence, leave
the choice so that CC253-based boards not using this option can build and work
fine.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge pull request #463 from ADVANSEE/cc2538-ports-pins

cc2538: Clean up and improve port and pin definitions