lib: Add xxhash support

This adds the xxhash support from Linux. Files are almost identical to
those added to Linux in commit 5d240522 ("lib: Add xxhash module") (they
haven't been touched since in Linux). The only difference is to add some
includes to be compatible with U-Boot. Also SPDX lincese tags were
added.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/lib/Kconfig b/lib/Kconfig
index 3801250..1ed69fa 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -327,6 +327,9 @@
 config CRC32C
 	bool
 
+config XXHASH
+	bool
+
 endmenu
 
 menu "Compression Support"