"target/foo/cpu.h" contains the target specific declarations.

A heterogeneous setup need to access target agnostic declarations
(at least the QOM ones, to instantiate the objects).

Our convention is to add such target agnostic QOM declarations in
the "target/foo/cpu-qom.h" header.
Add a comment clarifying that in the header.

Extract QOM definitions from "cpu.h" to "cpu-qom.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Brian Cain <bcain@quicinc.com>
Message-Id: <20231013140116.255-8-philmd@linaro.org>