lib: add Zstandard decompression support

Add the zstd library from Linux kernel (only decompression support).
There are minimal changes to build with U-Boot, otherwise the files are
identical to Linux commit dc35da16 from March 2018, the files had not
been touched since in kernel. Also SPDX lincese tags were added.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/lib/zstd/Makefile b/lib/zstd/Makefile
new file mode 100644
index 0000000..33c1df4
--- /dev/null
+++ b/lib/zstd/Makefile
@@ -0,0 +1,4 @@
+obj-y += zstd_decompress.o
+
+zstd_decompress-y := huf_decompress.o decompress.o \
+		     entropy_common.o fse_decompress.o zstd_common.o