arm64: hyp: initialize vttbr_el2 to zero

The architecture doesn't mandate any reset value for vttbr_el2.
Better set it to a known value before some HYP code gets confused.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>