gpio: of_get_named_gpio_flags() return -EPROBE_DEFER if GPIO not yet available

of_get_named_gpio_flags() and of_get_named_gpio() return -EPROBE_DEFER if the
respective GPIO is not (yet) available. This is useful if driver's probe()
functions try to get a GPIO whose controller isn't probed yet. Thus, the driver
can be probed again later on.

The function still returns -EINVAL on other errors (parse error or node doesn't
exist). This way, the case of an optional/intentionally missing GPIO is handled
appropriately.

Change-Id: Icf8026b9af2192a287d8609df1054ad4d1a14658
Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[stepanm@codeaurora.org: backported to 3.4]
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Patch-mainline: v3.6
Git-commit: 4fbb0022

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>