selftests/powerpc: Add test for explicitly changing DSCR value

This patch adds a test which modifies the DSCR using mtspr instruction
and verifies the change using mfspr instruction. It uses both the
privilege state SPR as well as the problem state SPR for the purpose.

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>