ARM: qemu-arm: Add support for AArch64

This adds support for '-machine virt' on AArch64. This is rather simple:
we just add TARGET_QEMU_ARM_xxBIT to select a few different Kconfig
symbols, provide the ARMv8 memory map from the board file and add a new
defconfig based on the 32-bit defconfig.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Tom Rini <trini@konsulko.com>