nand: zynq: Add support for 16-bit buswidth

This patch adds support for 16-bit buswidth by determining
the bus width based on mio configuration.

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