mtd: rawnand: sunxi: Remove support for GPIO-based Ready/Busy polling

None of the existing platforms connect the R/B pin to a GPIO (they all
use one of the dedicated R/B pin).
Anyway, if we ever get short of native R/B pins, it's probably better
to fallback to STATUS reg polling than trying to poll a GPIO.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>