pinctrl: OF: Don't create a pinctrl handle if no pinctrl entries exist

When pinctrl_get() is called for a device, it will return a valid handle
even if the device itself has no pinctrl state entries defined in
device-tree. This is caused by the function pinctrl_dt_to_map() which
will return success even if the first pinctrl state, 'pinctrl-0', is not
found in the device-tree node for a device.

According to the pinctrl device-tree binding documentation, pinctrl
states must be numbered starting from 0 and so 'pinctrl-0' should always
be present if a device uses pinctrl and therefore, if 'pinctrl-0' is not
present it seems valid that we should not return a valid pinctrl handle.

Fix this by returning an error code if the property 'pinctrl-0' is not
present for a device.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>