ARM: virt: hide CONFIG_ARM_VIRT_EXT from user

ARM_VIRT_EXT is a property of CPU_V7, but does not adversely affect
other CPUs that can be built into the same kernel image (i.e. ARMv6+).

This patch defaults ARM_VIRT_EXT to y if CPU_V7, allowing hypervisors
such as KVM to make better use of the option and being able to rely
on hyp-mode boot support.

Signed-off-by: Will Deacon <will.deacon@arm.com>