Documentation/powerpc: Add proper links for manual and tests

The links that are mentioned in this document are no longer
valid. So changed the proper links for NXGZIP user manual and
test cases.

Reported-by: Bulent Abali <abali@us.ibm.com>
Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/08511c1e92ac239f20ac88c73c59d1f8cf02e6ad.camel@linux.ibm.com