You can use "void pmc_enable_sleepmode(uint8_t uc_type)" library call to enter into sleep mode. It prepares the PMC_FSMR register for sleep mode, then if "uc_type" is zero, the it executes "__WFI()" otherwise it executes "__WFE()".
I tested it with parameter zero ("__WFI()"): it worked as expected, the MCU went into sleep, then serial interrupts, the timer interrupt and any other activated interrupts waked up the MCU.