xtensa: do not enable GENERIC_GPIO by default

Now that drivers/gpio/devres.c build does not depend on GPIOLIB do not
enable GENERIC_GPIO by default to fix the following build errors seen
in the linux-next:

include/asm-generic/gpio.h:270:2: error: implicit declaration of function
	'__gpio_get_value' [-Werror=implicit-function-declaration]
include/asm-generic/gpio.h:276:2: error: implicit declaration of function
	'__gpio_set_value' [-Werror=implicit-function-declaration]
include/linux/gpio.h:60:19: error: redefinition of 'gpio_cansleep'
	include/linux/gpio.h:62:2: error: implicit declaration of function
       	'__gpio_cansleep' [-Werror=implicit-function-declaration]
include/linux/gpio.h:67:2: error: implicit declaration of function
	'__gpio_to_irq' [-Werror=implicit-function-declaration]
drivers/gpio/devres.c:26:2: error: implicit declaration of function
	'gpio_free' [-Werror=implicit-function-declaration]
drivers/gpio/devres.c:60:2: error: implicit declaration of function
	'gpio_request' [-Werror=implicit-function-declaration]
drivers/gpio/devres.c:90:2: error: implicit declaration of function
	'gpio_request_one' [-Werror=implicit-function-declaration]

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>