pinctrl.txt: Remove outdated information

Returning -EAGAIN is no longer supported by pin_config_group_set()
since ad42fc6c ("pinctrl: rip out the direct pinconf API")

Remove the relevant section from the documentation.
Signed-off-by: Ramon Fried <ramon.fried@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/pinctl.rst

@@ -274,15 +274,6 @@ configuration in the pin controller ops like this::
 		.confops = &foo_pconf_ops,
 	};
 
-Since some controllers have special logic for handling entire groups of pins
-they can exploit the special whole-group pin control function. The
-pin_config_group_set() callback is allowed to return the error code -EAGAIN,
-for groups it does not want to handle, or if it just wants to do some
-group-level handling and then fall through to iterate over all pins, in which
-case each individual pin will be treated by separate pin_config_set() calls as
-well.
-
-
 Interaction with the GPIO subsystem
 ===================================