coreboot: Add qemu_arm64_12mb board

Add a build for QEMU AArch64 virtual machine using U-Boot as payload.
Coreboot config is based on the following defconfig:

    CONFIG_CBFS_SIZE=0x00c00000
    CONFIG_BOARD_EMULATION_QEMU_AARCH64=y
    CONFIG_CONSOLE_CBMEM_BUFFER_SIZE=0x20000
    CONFIG_COREBOOT_ROMSIZE_KB_12288=y
    CONFIG_UART_PCI_ADDR=0x0

The resulting ROM can be booted with a command line like:

    qemu-system-aarch64 \
        -machine virt,secure=on,virtualization=on \
        -cpu cortex-a53 -m 1G \
        -vga none -display none -serial stdio \
        -bios bin/qemu_arm64_12mb/uboot_*.rom

However, this is little more than a proof of concept because U-Boot
upstream is missing coreboot integration on non-x86 boards, which could
have been useful for e.g. a framebuffer.

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