ARM: zynq: slcr: Dont modify the reserved bits

Set only the 0-3 bits of the FPGA_RST_CTRL register
as other bits should not be set to 1.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>