ARM: virt: make mach-virt just a kconfig option

The mach code for mach-virt is no longer needed, so we can remove all of
mach-virt except the kconfig entry.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>