gpiolib: Introduce GPIO_LOOKUP_FLAGS_DEFAULT

Since GPIO library operates with enumerator when it's subject to handle
the GPIO lookup flags, it will be better to clearly see what default means.

Thus, introduce GPIO_LOOKUP_FLAGS_DEFAULT entry to describe
the default assumptions.

While here, replace 0 by newly introduced constant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>