uboot-at91: Add PKG_MIRROR_HASH to fix download

The referenced commit is gone, but we already have this file on our
mirror, use that one by providing the correct mirror hash.

I generated a tar.xz file with the given git commit hash using a random
fork on github and it generated the same tar.xz file as found on our
mirror so this looks correct.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>