arm64: vdso: constify vm_special_mapping used for aarch32 vectors page

The vm_special_mapping spec which is used for aarch32 vectors page is
never modified, so mark it as const.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>