mtd: nand: jz4740: Convert to GPIO descriptor API

Use the GPIO descriptor API instead of the deprecated legacy GPIO API to
manage the busy GPIO.

The patch updates both the jz4740 nand driver and the only user of the driver
the qi-lb60 board driver.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>