arm64: Allow 48-bits VA space without ARM_SMMU

Now when KVM has been reworked to support 48-bits host VA space, we can
allow systems to be configured with this option.  However, the ARM SMMU
driver also needs to be tweaked for 48-bit support so only allow the
config option to be set when not including support for theSMMU.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>