mtd: gpio-nand: Ability to use driver for configurations without RDY-pin

In some configurations of "gpio-nand" RDY-pin may be not connected.
This patch allow to use driver for these configurations. In this case
we are assume that device always ready.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>