posix: clock: implement clock_getres()

clock_getres() is required by the POSIX_TIMERS Option Group
as detailed in Section E.1 of IEEE-1003.1-2017.

The POSIX_TIMERS Option Group is required for PSE51, PSE52,
PSE53, and PSE54 conformance, and is otherwise mandatory for
any POSIX conforming system as per Section A.2.1.3 of
IEEE-1003-1.2017.

With this, we have complete support for the POSIX_TIMERS
Option Group.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>