zynq: slcr: Disable all level shifters

Disable all level shifters before enabling
the PS-to-PL level shifters as it would
be good to disable all level shifters before
enabling the PS-to-PL in order to ensure that
it is in proper state

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>