arm64: efi: replace open coded constants with symbolic ones

Replace open coded constants with symbolic ones throughout the
Image and the EFI headers. No binary level changes are intended.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>