armv8: update gd after relocate

We need to update gd in assamble code after relocate,
this is a fix to:
adc421e arm: move gd handling outside of C code

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

arch/arm/lib/crt0_64.S

@@ -120,6 +120,8 @@ relocation_return:
 #endif /* !CONFIG_SPL_BUILD */
 #if defined(CONFIG_SPL_BUILD)
 	bl	spl_relocate_stack_gd           /* may return NULL */
+	/* set up gd here, outside any C code */
+	mov	x18, x0
 	/*
 	 * Perform 'sp = (x0 != NULL) ? x0 : sp' while working
 	 * around the constraint that conditional moves can not