arm64: vdso: add __init section marker to alloc_vectors_page

It is not needed after booting, this patch moves the alloc_vectors_page
function to the __init section.

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>