zynq: timer: Fix hangs if network activity attempted after about one hour

zynq: timer: Fix hangs if network activity attempted after about one hour

Cortex-A9 MPCore TRM' from ARM (ARM DDI 0407G ID072711) describes
in the section 4.1.1 how this value calculation should be done.

This patch fixes the problem if network activity such as ping or
tftp is attempted after u-boot has been idle for an hour,
it hangs, and cannot control-C out of it.

Signed-off-by: Uday Hegde <udayh@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>