selftests/powerpc/dexcr: Add DEXCR config details to lsdexcr

Now that the DEXCR can be configured with prctl, add a section in
lsdexcr that explains why each aspect is set the way it is.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240417112325.728010-8-bgray@linux.ibm.com