cc2538: lpm: Fix build for LPM_CONF_ENABLE == 0

lpm.c needs to #include lpm.h in order to get the definition of
lpm_periph_permit_pm1_func_t, which made the replacement macros conflict with
the function definitions for the LPM_CONF_ENABLE == 0 case. This change fixes
this issue by #if-ing out the code in lpm.c in that case. Also, the replacement
macro for lpm_register_peripheral() was missing in that case, which is fixed
here.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge pull request #546 from ADVANSEE/cc2538-fix-disabled-lpm

cc2538: lpm: Fix build for LPM_CONF_ENABLE == 0