gpio: gpiolib: Generalise state persistence beyond sleep

General support for state persistence is added to gpiolib with the
introduction of a new pinconf parameter to propagate the request to
hardware. The existing persistence support for sleep is adapted to
include hardware support if the GPIO driver provides it. Persistence
continues to be enabled by default; in-kernel consumers can opt out, but
userspace (currently) does not have a choice.

The *_SLEEP_MAY_LOSE_VALUE and *_SLEEP_MAINTAIN_VALUE symbols are
renamed, dropping the SLEEP prefix to reflect that the concept is no
longer sleep-specific.  I feel that renaming to just *_MAY_LOSE_VALUE
could initially be misinterpreted, so I've further changed the symbols
to *_TRANSITORY and *_PERSISTENT to address this.

The sysfs interface is modified only to keep consistency with the
chardev interface in enforcing persistence for userspace exports.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/pinctrl/pinconf-generic.h

@@ -94,6 +94,7 @@
  *	or latch delay (on outputs) this parameter (in a custom format)
  *	specifies the clock skew or latch delay. It typically controls how
  *	many double inverters are put in front of the line.
+ * @PIN_CONFIG_PERSIST_STATE: retain pin state across sleep or controller reset
  * @PIN_CONFIG_END: this is the last enumerator for pin configurations, if
  *	you need to pass in custom configurations to the pin controller, use
  *	PIN_CONFIG_END+1 as the base offset.
@@ -122,6 +123,7 @@ enum pin_config_param {
 	PIN_CONFIG_SLEEP_HARDWARE_STATE,
 	PIN_CONFIG_SLEW_RATE,
 	PIN_CONFIG_SKEW_DELAY,
+	PIN_CONFIG_PERSIST_STATE,
 	PIN_CONFIG_END = 0x7F,
 	PIN_CONFIG_MAX = 0xFF,
 };