u-boot: Add qemu_arm64_12mb board

Add a U-Boot payload build for the QEMU AArch64 virtual machine. The
config is same as upstream "qemu-arm64" defconfig, but SYS_TEXT_BASE is
set to 0x50000000 so that it doesn't conflict with coreboot. QEMU
auto-generates and passes a device-tree file to U-Boot at runtime,
there's no compile-time canonical version, so there's no need to set
REMAKE_ELF or OF_EMBED.

It's not immediately obvious if QEMU-specific drivers are available to
support display output, but most coreboot integration is unavailable
(depends on x86) and entire video subsystem is disabled in the U-Boot
upstream defconfig.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

resources/u-boot/qemu_arm64_12mb/board.cfg

+ubtree="default"
+ubrevision="4debc57a3da6c3f4d3f89a637e99206f4cea0a96" # v2022.10
+arch="AArch64"