fsl-layerscape: Add RESV_RAM check in resv_ram addr

The initialization of gd->arch.resv_ram pointer should depend on if the
RESV_RAM config is enabled.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Wasim Khan <wasim.khan@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>