mtd: zynq: nand: Move board_nand_init() function to board.c

Putting board_nand_init() function inside NAND driver was not appropriate
due to it doesn't allow board vendor to customise their NAND
initialization code such as adding NAND lock/unlock code.

This commit was to move the board_nand_init() function from NAND driver
to board.c file. This allow customization of board_nand_init() function.

Signed-off-by: Wilson Lee <wilson.lee@ni.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Keng Soon Cheah <keng.soon.cheah@ni.com>
Cc: Chen Yee Chew <chen.yee.chew@ni.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Cc: Scott Wood <oss@buserror.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>