selftests/powerpc/dexcr: Add DEXCR prctl interface test

Some basic tests of the prctl interface of the DEXCR.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
[mpe: Add missing SPDX tag]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240417112325.728010-6-bgray@linux.ibm.com