ppc: spapr: cleanup cr get/set with helpers.

ppc: spapr: cleanup cr get/set with helpers.

The bits in cr reg are grouped into eight 4-bit fields represented
by env->crf[8] and the related calculations should be abstracted to
keep the calling routines simpler to read. This is a step towards
cleaning up the related/calling code for better readability.

Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230503093619.2530487-2-harshpb@linux.ibm.com>
[danielhb: add 'const' modifier to fix linux-user build]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>