zstd: Create a function for use from U-Boot

The existing zstd API requires the same sequence of calls to perform its
task. Create a helper for U-Boot, to avoid code duplication, as is done
with other compression algorithms. Make use of of this from the image
code.

Note that the zstd code lacks a test in test/compression.c and this should
be added by the maintainer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/zstd/Makefile b/lib/zstd/Makefile
index 33c1df4..1217089 100644
--- a/lib/zstd/Makefile
+++ b/lib/zstd/Makefile
@@ -1,4 +1,4 @@
 obj-y += zstd_decompress.o
 
 zstd_decompress-y := huf_decompress.o decompress.o \
-		     entropy_common.o fse_decompress.o zstd_common.o
+		     entropy_common.o fse_decompress.o zstd_common.o zstd.o