cc2538: lpm: Make periph_permit_pm1_funcs static

periph_permit_pm1_funcs[] is a private member of lpm.c, so define it as static.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge pull request #433 from ADVANSEE/cc2538-lpm-static-periph_permit_pm1_funcs

cc2538: lpm: Make periph_permit_pm1_funcs static