armv8/fsl-layerscape: Remove reference to gdata

The global_data pointer (gd) has been set earlier in crt0_64.S.
So there's no need to assign it again. Remove gdata since it is going
away in U-Boot.

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>