arm-trusted-firmware-sunxi: bump to v2.3 and use atf-builds binaries

arm-trusted-firmware-sunxi: bump to v2.3 and use atf-builds binaries

Update ATF to latest v2.3 release and use reproducible bl31 binaries
from atf-builds project.

atf-builds project started recently offering reproducible ATF binaries
which is more convenient then building very tiny ATF artifacts from
scratch during each build, thus saving considerable build resources.  It
has reduced build from scratch time on my CI builder for sunxi/cortexa53
target from 57min down to 29min.

Run tested on a64-olinuxino-emmc.

 U-Boot SPL 2020.04 (Apr 20 2020 - 08:28:05 +0000)
 DRAM: 1024 MiB
 Trying to boot from MMC1
 NOTICE:  BL31: v2.2():v2.3-rc1
 NOTICE:  BL31: Built : 19:20:57, Apr 15 2020
 NOTICE:  BL31: Detected Allwinner A64/H64/R18 SoC (1689)
 NOTICE:  BL31: Found U-Boot DTB at 0x4090dc0, model: Olimex A64-Olinuxino-eMMC

Cc: Zoltan HERPAI <wigyori@uid0.hu>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Petr Å tetiar <ynezz@true.cz>