armv8: spl: Fix build with LINUX_KERNEL_IMAGE_HEADER

Setting LINUX_KERNEL_IMAGE_HEADER=y attempts to include an ARM64 Linux
kernel image header at the start of both U-Boot proper and SPL binaries.
However, some definitions that the image header uses are not included by
the SPL linker script, resulting in a build error. Include them the way
they are included in U-Boot proper's linker script to fix the error.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>