arm64: move SEGMENT_ALIGN to <asm/memory.h>

Currently we define SEGMENT_ALIGN directly in our vmlinux.lds.S.

This is unfortunate, as the EFI stub currently open-codes the same
number, and in future we'll want to fiddle with this.

This patch moves the definition to our <asm/memory.h>, where it can be
used by both vmlinux.lds.S and the EFI stub code.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>