nand: zynq: Cleanup initialization

nand: zynq: Cleanup initialization

CONFIG_NAND_ZYNQ selects CONFIG_SYS_NAND_SELF_INIT, so the
driver doesn't have to play any ifdef game.

Also, we can mark zynq_nand_init() as static and get rid
of the mach-specific nand.h header.

This is really a revert of:
"mtd: zynq: nand: Move board_nand_init() function to board.c"
(sha1: 310995d9f91ae56082b49be06fe8c3d01424f8f6)

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>