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/Makefile b/lib/Makefile
index 47829bf..6a52543 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -37,6 +37,7 @@
 obj-$(CONFIG_IMAGE_SPARSE) += image-sparse.o
 obj-y += ldiv.o
 obj-$(CONFIG_MD5) += md5.o
+obj-$(CONFIG_XXHASH) += xxhash.o
 obj-y += net_utils.o
 obj-$(CONFIG_PHYSMEM) += physmem.o
 obj-y += rc4.o