ath79: add support for reset key on MikroTik RB912UAG-2HPnD

On MikroTik RB91x board series a reset key shares SoC gpio
line #15 with NAND ALE and NAND IO7. So we need a custom
gpio driver to manage this non-trivial connection schema.
Also rb91x-nand needs to have an ability to disable a polling
of the key while it works with NAND.

While we've been integrating rb91x-key into a firmware, we've
figured out that:
* In the gpio-latch driver we need to add a "cansleep" suffix to
several gpiolib calls,
* When gpio-latch and rb91x-nand fail to get a gpio and an error
is -EPROBE_DEFER, they shouldn't report about this, since this
actually is not an error and occurs when the gpio-latch probe
function is called before the rb91x-key probe.
We fix these related things here too.

Signed-off-by: Denis Kalashnikov <denis281089@gmail.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>