selftests/powerpc: add const qualification where possible

Various char * parameters in the common powerpc selftest APIs can be
const.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230817-powerpc-selftest-misc-v1-2-a84cc1ef78b2@linux.ibm.com