pinctrl: rip out the direct pinconf API

From the inception ot the pin config API there has been the
possibility to get a handle at a pin directly and configure
its electrical characteristics. For this reason we had:

int pin_config_get(const char *dev_name, const char *name,
               unsigned long *config);
int pin_config_set(const char *dev_name, const char *name,
               unsigned long config);
int pin_config_group_get(const char *dev_name,
               const char *pin_group,
               unsigned long *config);
int pin_config_group_set(const char *dev_name,
               const char *pin_group,
               unsigned long config);

After the introduction of the pin control states that will
control pins associated with devices, and its subsequent
introduction to the device core, as well as the
introduction of pin control hogs that can set up states on
boot and optionally also at sleep, this direct pin control
API is a thing of the past.

As could be expected, it has zero in-kernel users.
Let's delete this API and make our world simpler.

Reported-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>