QOM objects shouldn't access each other internals fields
except using the QOM API.

mips_cps_realize() instantiates a TYPE_MIPS_ITU object, and
directly sets the 'saar' pointer:

   if (saar_present) {
       s->itu.saar = &env->CP0_SAAR;
   }

In order to avoid that, pass the MIPS_CPU object via a QOM
link property, and set the 'saar' pointer in mips_itu_realize().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20230203113650.78146-10-philmd@linaro.org>