Marek Behún authored Apr 29, 2019 and Tom Rini committed May 05, 2019

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>