gprs: Add GPRS timer conversion functions

Currently, all GPRS timer values are hard-coded. To make these values
configurable in seconds and to show them, conversion functions from
and to seconds are needed.

This patch adds gprs_tmr_to_secs and gprs_secs_to_tmr_floor. Due to
the limited number of bits used to encode GPRS timer values, only a
few durations can be represented. gprs_secs_to_tmr_floor therefore
always returns the timer value that represents either the exact
number (if an exact representation exists) or the next lower number
for that an exact representation exists.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gprs/gprs_test.ok b/openbsc/tests/gprs/gprs_test.ok
index 4825b67..cf71076 100644
--- a/openbsc/tests/gprs/gprs_test.ok
+++ b/openbsc/tests/gprs/gprs_test.ok
@@ -27,4 +27,5 @@
   Testing Purge MS Request
   Testing Purge MS Error
   Testing Purge MS Result
+Test GPRS timer decoding/encoding
 Done.