cc2538: gpio: Fix usage of parameters in macros

The parameters in the GPIO macros were used without being parenthesized. This
could generate wrong values for register assignments in the case of expressions
passed as arguments to these macros.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: gpio: Add pin read / write and interrupt clear macros

Introduce new useful GPIO macros to:
 - read the levels of some port pins,
 - write the levels of some port pins (pass bit-field value to be set),
 - clear the interrupt flags for some port pins.

These macros are cleaner and less error prone than raw register access code
copied all over the place.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: gpio: Use accessor macros

Use the GPIO accessor macros instead of copying raw register access code all
over the place. This is cleaner and less error prone.

This fixes the setting of the USB pull-up resistor that worked only by chance on
the CC2538DK because it is controlled by the pin 0 of the used GPIO port.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge pull request #425 from ADVANSEE/cc2538-gpio

cc2538: gpio: Bug fix and various improvements