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 Behún <marek.behun@nic.cz>