i2c: document generic DT bindings for timing parameters

Also, sort the properties alphabetically and make indentation
consistent. Wording largely taken from i2c-rk3x.txt, thanks guys!

Only "i2c-scl-internal-delay-ns" is new, the rest is used by two drivers
already and was documented in their driver binding documentation.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c.txt

@@ -29,12 +29,33 @@ Optional properties
 These properties may not be supported by all drivers. However, if a driver
 wants to support one of the below features, it should adapt the bindings below.
 
-- clock-frequency	- frequency of bus clock in Hz.
-- wakeup-source		- device can be used as a wakeup source.
+- clock-frequency
+	frequency of bus clock in Hz.
 
-- interrupts		- interrupts used by the device.
-- interrupt-names	- "irq" and "wakeup" names are recognized by I2C core,
-			  other names are left to individual drivers.
+- i2c-scl-falling-time-ns
+	Number of nanoseconds the SCL signal takes to fall; t(f) in the I2C
+	specification.
+
+- i2c-scl-internal-delay-ns
+	Number of nanoseconds the IP core additionally needs to setup SCL.
+
+- i2c-scl-rising-time-ns
+	Number of nanoseconds the SCL signal takes to rise; t(r) in the I2C
+	specification.
+
+- i2c-sda-falling-time-ns
+	Number of nanoseconds the SDA signal takes to fall; t(f) in the I2C
+	specification.
+
+- interrupts
+	interrupts used by the device.
+
+- interrupt-names
+	"irq" and "wakeup" names are recognized by I2C core, other names are
+	left to individual drivers.
+
+- wakeup-source
+	device can be used as a wakeup source.
 
 Binding may contain optional "interrupts" property, describing interrupts
 used by the device. I2C core will assign "irq" interrupt (or the very first