posix: timer: unable to create a timer with CLOCK_REALTIME

[cfriedt]

Describe the bug
According to the spec for timer_create()

All implementations shall support a clock_id of CLOCK_REALTIME

However, currently, attempting to create a timer using that clock id results in -1 / EINVAL.

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? Any
• What have you tried to diagnose or workaround this issue? Looked at code
• Is this a regression? Techniically no, but the bug was introduced in f603e60 (where timers were added)

To Reproduce
Steps to reproduce the behavior:

1. Look at code
2. Compare with spec
3. See error

Expected behavior
Should be able to use CLOCK_REALTIME with timer_create().

Impact
Without this fix, conformance is broken

Logs and console output

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK v0.16.4
• Commit: f603e60

Additional context