vexpress64: fix a typo of SPDX-License-Identifier

Misspelling of SPDX-License-Identifier is rather fatal than other
general typos, so must be fixed.

This file spells SPDX-Licence-Identifier.
                           ^

I also moved it to the very top of the file with // comment style.

Detected by grepping the source tree:

$ git grep --not -e SPDX-License-Identifier --and -e SPDX-
board/armltd/vexpress64/pcie.c: * SPDX-Licence-Identifier:      GPL-2.0+

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Liviu Dudau <liviu.dudau@foss.arm.com>