arm-trusted-firmware-mediatek: update to git HEAD

Most notably this enabled use of pstore/ramoops on MT7622 as DRAM
content is now preserved over reboot.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/boot/arm-trusted-firmware-mediatek/Makefile

@@ -13,9 +13,9 @@ PKG_RELEASE:=$(AUTORELEASE)
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL=https://github.com/mtk-openwrt/arm-trusted-firmware.git
-PKG_SOURCE_DATE:=2021-04-28
-PKG_SOURCE_VERSION:=a63914612904642ed974390fff620f7003ebc20a
-PKG_MIRROR_HASH:=6396a36406ceecf667470d0d673552751573de65497bf97d1e2af3505fe79366
+PKG_SOURCE_DATE:=2021-05-08
+PKG_SOURCE_VERSION:=d2c75b2139be003887af9cc5a94da5e9bdc59de7
+PKG_MIRROR_HASH:=4af9ce8e11511afee7f588cc982946c06339edbfa47afef6a7f3e2231ac9f34d
 
 PKG_MAINTAINER:=Daniel Golle <daniel@makrotopia.org>