selftests/powerpc: Add NX-GZIP engine decompress testcase

Include a decompression testcase for the powerpc NX-GZIP
engine.

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/20200420205538.25181-5-rzinsly@linux.ibm.com