[POWERPC] Use mktime in timer sysdev

This makes the timer sysdev use mktime instead of rtc_tm_to_time,
since rtc_tm_to_time just calls mktime anyway, and this means we
don't have a dependency on rtc-lib.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>