microblaze: Do not select GENERIC_GPIO by default

The microblaze architecture does not provide a native GPIO API implementation
nor requires GPIOLIB, but still selects GENERIC_GPIO by default. As a result the
following build error occurs, if GPIOLIB is not selected:

	include/asm-generic/gpio.h: In function 'gpio_get_value_cansleep':
	include/asm-generic/gpio.h:218: error: implicit declaration of function '__gpio_get_value'
	include/asm-generic/gpio.h: In function 'gpio_set_value_cansleep':
	include/asm-generic/gpio.h:224: error: implicit declaration of function '__gpio_set_value'

This patch addresses the issue by not selecting GENERIC_GPIO by default. This
causes the GPIO API to be stubbed out if no implementation is provided.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: stable@vger.kernel.org
Tested-by: Michal Simek <monstr@monstr.eu>