test: posix: xsi_realtime: do not exclude intel_ish platforms

Previously, intel_ish platforms would fail to link due to
missing `arch_mem_map()` and `arch_mem_unmap()` symbols due
to nuances in the x86 architecture on those platforms.

A separate commit has made MMU a non-user-configurable option,
which removes the ambiguity in POSIX.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>