selftests/powerpc: Add test for system wide DSCR default

This patch adds a test case for the system wide DSCR default value,
which when changed through it's sysfs interface must be visible to all
threads reading DSCR either through the privilege state SPR or the
problem state SPR. The DSCR value change should be immediate as well.

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>