armv8: u-boot-spl.lds: mark __image_copy_start as symbol

In arch/arm/lib/sections.c there is below code:
char __image_copy_start[0] __section(".__image_copy_start");
But actually 'objdump -t spl/u-boot-spl' not able to find out
symbol '__image_copy_start' for binman update image-pos/size.

So update link file

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>