powerpc/dscr: Add documentation for DSCR support

This patch adds a new documentation file explaining the DSCR support on
powerpc platforms. This explains DSCR related data structure, code paths
and also available user interfaces. Any further functional changes to
the DSCR support in the kernel should definitely update the
documentation here.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>