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>

cpu/cc2538/lpm.h

@@ -126,6 +126,12 @@ void lpm_init(void);
  * lpm_exit(), which will always be called from within the Sleep Timer ISR
  * context.
  *
+ * \note Dropping to PM2 means that data in the SRAM non-retention area will
+ * be lost. It is recommended to disable PM2 if the total RAM footprint is
+ * larger than what will fit in the retention area.
+ * .nrdata* sections can be used to place uninitialized data in the SRAM
+ * non-retention area.
+ *
  * \sa main(), rtimer_arch_next_trigger(), lpm_exit(), lpm_set_max_pm()
  */
 void lpm_enter(void);