ARM: KVM: enable KVM in Kconfig on big-endian systems

Previous patches addresses ARMV7 big-endian virtualiztion,
kvm related issues, so enable ARM_VIRT_EXT for big-endian
now.

Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>