get_cycles() is used in some places as one part of the seed for the
kernel PRNG.
This backports the following commit from mainline linux kernel:
9c9b415c50bc298ac61412dff856eae2f54889ee
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38834 3c298f89-4303-0410-b956-a3cf2f4a3e73