selftests/powerpc: Add test for DSCR SPR numbers

This patch adds a test which verifies that the DSCR privilege and
problem state SPR read & write accesses while making sure that the
results are always the same irrespective of which SPR number is being
used.

Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>