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

tools/testing/selftests/powerpc/nx-gzip/99-nx-gzip.rules

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+SUBSYSTEM=="nxgzip", KERNEL=="nx-gzip", MODE="0666"

tools/testing/selftests/powerpc/nx-gzip/README

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+Test the nx-gzip function:
+=========================
+
+Verify that following device exists:
+  /dev/crypto/nx-gzip
+If you get a permission error run as sudo or set the device permissions:
+   sudo chmod go+rw /dev/crypto/nx-gzip
+However, chmod may not survive across boots. You may create a udev file such
+as:
+   /etc/udev/rules.d/99-nx-gzip.rules
+
+
+To manually build and run:
+$ gcc -O3 -I./include -o gzfht_test gzfht_test.c gzip_vas.c
+$ gcc -O3 -I./include -o gunz_test gunz_test.c gzip_vas.c
+
+
+Compress any file using Fixed Huffman mode. Output will have a .nx.gz suffix:
+$ ./gzfht_test gzip_vas.c
+file gzip_vas.c read, 6413 bytes
+compressed 6413 to 3124 bytes total, crc32 checksum = abd15e8a
+
+
+Uncompress the previous output. Output will have a .nx.gunzip suffix:
+./gunz_test gzip_vas.c.nx.gz
+gzHeader FLG 0
+00 00 00 00 04 03
+gzHeader MTIME, XFL, OS ignored
+computed checksum abd15e8a isize 0000190d
+stored   checksum abd15e8a isize 0000190d
+decomp is complete: fclose
+
+
+Compare two files:
+$ sha1sum gzip_vas.c.nx.gz.nx.gunzip gzip_vas.c
+bf43e3c0c3651f5f22b6f9784cd9b1eeab4120b6  gzip_vas.c.nx.gz.nx.gunzip
+bf43e3c0c3651f5f22b6f9784cd9b1eeab4120b6  gzip_vas.c
+
+
+Note that the code here are intended for testing the nx-gzip hardware function.
+They are not intended for demonstrating performance or compression ratio.
+By being simplistic these selftests expect to allocate the entire set of source
+and target pages in the memory so it needs enough memory to work.
+For more information and source code consider using:
+https://github.com/libnxz/power-gzip