zynq: nand: Runtime detection of nand buswidth through slcr

This patch adds support to check the buswidth on nand flash
at runtime based on nand MIO configurations done by FSBL.

User needs to correctly configure the MIO's based on the
buswidth supported by the nand flash which is present on the board.

Added nand8 and nand16 @periph names on slcr driver.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>