mtd: atmel_nand: fix deferred probe from __init

Move probe out of __init section and don't use platform_driver_probe
which cannot be used with deferred probing.

Since commit e9354576 ("gpiolib: Defer failed gpio requests by default")
this driver might return -EPROBE_DEFER if a gpio_request fails.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>