arm64/efi: set PE/COFF file alignment to 512 bytes

Change our PE/COFF header to use the minimum file alignment of
512 bytes (0x200), as mandated by the PE/COFF spec v8.3

Also update the linker script so that the Image file itself is also a
round multiple of FileAlignment.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>