cc2538: lpm: Give access to the SRAM non-retention area for PM2

If PM2 is enabled with LPM_CONF_MAX_PM, but not active, the non-retention area
of the SRAM can be useful to place temporary data that does not fit in the
low-leakage SRAM, typically after having called lpm_set_max_pm(LPM_PM1). Hence,
give access to this non-retention area thanks to .nrdata* sections.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538dk: Make it possible to override SLIP_ARCH_CONF_ENABLED

As the comment in contiki-conf.h says, the automatic definition of
SLIP_ARCH_CONF_ENABLED works only if UIP_FALLBACK_INTERFACE is tied to SLIP. If
UIP_FALLBACK_INTERFACE is set to another interface, SLIP_ARCH_CONF_ENABLED is
still automatically set to 1, leading to unwanted SLIP_END characters from
dbg.c:putchar() being printed on the UART.

This change makes it possible to force the definition of SLIP_ARCH_CONF_ENABLED
(e.g. from project-conf.h), so that it can be disabled if UIP_FALLBACK_INTERFACE
is used with something else than SLIP.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
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 #466 from ADVANSEE/cc2538dk-slip-enable

cc2538dk: Make it possible to override SLIP_ARCH_CONF_ENABLED
Merge pull request #464 from ADVANSEE/cc2538-pm2-sram

cc2538: lpm: Give access to the SRAM non-retention area for PM2
Merge pull request #471 from ADVANSEE/cc2538-gpio-power-up

cc2538: gpio: Add support for power up