arm: qemu: implement enable_caches()

Add an override for enable_caches to enable the I and D caches, along
with the cached 1:1 mapping of all of DRAM. This is needed for running
U-Boot under virtualization with QEMU/kvm.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>