arm: zynq: Fix timer loadaddress

Reload address was written to the counter register
instead of load register.
The problem happens when timer expires but never
reload to ~0UL (it is downcount timer).

Reported-by: Stephen MacMahon <stephenm@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>