ARM: set GD_FLG_RELOC for boards skipping relocation to RAM

If CONFIG_SKIP_RELOCATE_UBOOT is set the flag GD_FLG_RELOC is usually
never set, because relocation to RAM is actually never done by U-boot
itself. However, several pieces of code check if this flag is set at
some time.

So, to make sure this flag is set on boards skipping relocation, this
is added to the initialisation of U-boot at a moment where it is safe
to do so.

Signed-off-by: Remy Bohmer <linux@bohmer.net>