plat: zynqmp: Check for DLL status before doing reset

plat: zynqmp: Check for DLL status before doing reset

This patch check for the DLL status before doing the DLL reset.
If DLL reset is already issued then skip the reset inside ATF
otherwise DLL reset will be issued.
By doing this way, all the following cases will be supported.
1. Patched ATF + Patched Linux base.
2. Older ATF + Patched Linux base.
3. Patched ATF + Older Linux base.

Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Change-Id: I53a0a27521330f1543275cc9cb44cd1dfc569c65