selftests/powerpc: Add test for all DSCR sysfs interfaces

This test continuously updates the system wide DSCR default value in the
sysfs interface and makes sure that the same is reflected across all the
sysfs interfaces for each individual CPUs present on the system.

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