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>
(cherry picked from commit 20a7c9d5c9d87595aa73ad39e95132df545a60ca)

package/boot/uboot-at91/Makefile

@@ -12,6 +12,7 @@ PKG_RELEASE:=1
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://github.com/linux4sam/u-boot-at91.git
+PKG_MIRROR_HASH:=03ae55f16e716c2c06481372174f9d8506fa077459bf321a024e0d23a9bb013a
 PKG_SOURCE_VERSION:=59f202622154f82e708a6ca2bf86350a5c1b2d33
 
 include $(INCLUDE_DIR)/u-boot.mk