mux: mmio: drop obsolete dependency on COMPILE_TEST

Since commit 0166dc11 ("of: make CONFIG_OF user selectable"), it
is possible to test-build any driver which depends on OF on any
architecture by explicitly selecting OF. Therefore depending on
COMPILE_TEST as an alternative is no longer needed.

It is actually better to always build such drivers with OF enabled,
so that the test builds are closer to how each driver will actually be
built on its intended target. Building them without OF may not test
much as the compiler will optimize out potentially large parts of the
code. In the worst case, this could even pop false positive warnings.
Dropping COMPILE_TEST here improves the quality of our testing and
avoids wasting time on non-existent issues.

As a minor optimization, this also lets us drop of_match_ptr(), as we
now know what it will resolve to, we might as well save cpp some work.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Peter Rosin <peda@axentia.se>