selftests/powerpc: Add README for GZIP engine tests

Include a README file with the instructions to use the
testcases at selftests/powerpc/nx-gzip.

Signed-off-by: Bulent Abali <abali@us.ibm.com>
Signed-off-by: Raphael Moreira Zinsly <rzinsly@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200420205538.25181-6-rzinsly@linux.ibm.com