cc2538: gpio: Add macros to use GPIO power-up interrupts

The GPIO power-up interrupts have to be configured and enabled in order to be
able to wake-up the SoC from PM1+ upon a signal edge occurring on a GPIO input
pin.

This set of macros allows to:
 - configure the signal edge triggering a power-up interrupt,
 - enable and disable a power-up interrupt,
 - clear a power-up interrupt flag.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: gpio: Clear the power-up interrupts in the port ISRs

The pending GPIO power-up interrupts have to be cleared in the ISRs in order not
to re-trigger the interrupts and the wake-up events.

The power-up interrupts of all pins are cleared for each port in the
corresponding port ISR. This is done after calling the registered callbacks so
that the callbacks can know which pin woke up the SoC. This is done after
clearing the regular interrupt in order to avoid getting a new wake-up interrupt
without the regular interrupt in the case of a new wake-up edge occurring
between the two clears.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge pull request #471 from ADVANSEE/cc2538-gpio-power-up

cc2538: gpio: Add support for power up