target/ppc: Introduce prot_for_access_type

Use this in the three places we currently have a local array
indexed by rwx (which happens to have the same values).
The types will match up correctly with additional changes.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210518201146.794854-2-richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>