soc/intel_adsp: Clean up cavs PWRCTL/PWRSTS/CLKCTL usage

These registers were defined in the new interface, but still being
used with bare bits (and in one spot a legacy field access macro).
Clean things up and use macros pervasively.  To be fair: in this
particular case the field names aren't particularly descriptive...

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>