DM: I2C: Introduce 'u-boot, i2c-transaction-bytes' property

The 'u-boot,i2c-transaction-bytes' device tree property provides
information regarding number of bytes transferred by a device in a
single transaction.

This change is necessary to avoid hanging devices after soft reset.
One notable example is communication with MC34708 device:

1. Reset when communicating with MC34708 via I2C.

2. The u-boot (after reboot -f) tries to setup the I2C and then calls
force_idle_bus. In the same time MC34708 still has some data to be sent
(as it transfers data in 24 bits chunks).

3. The force_idle_bus() is not able to make the bus idle as 8 SCL
clocks may be not enough to have the full transmission.

4. We end up with I2C inconsistency with MC34708.

This PMIC device requires 24+ SCL cycles to make finish any pending I2C
transmission.

Signed-off-by: Lukasz Majewski <lukma@denx.de>

doc/device-tree-bindings/i2c/i2c.txt

@@ -12,6 +12,10 @@ property which allows the chip offset length to be selected.
 Optional properties:
 - u-boot,i2c-offset-len - length of chip offset in bytes. If omitted the
     default value of 1 is used.
+- u-boot,i2c-transaction-bytes - the length of single I2C transaction on
+    the bus. Some devices require more than single byte transmission
+    (e.g. mc34708 mfd). This information is necessary to correctly
+     initialize (put into idle state) I2C bus after soft reset.
 - gpios = <sda ...>, <scl ...>;
   pinctrl-names = "default", "gpio";
   pinctrl-0 = <&i2c_xfer>;
@@ -28,6 +32,7 @@ i2c4: i2c@12ca0000 {
 		compatible = "google,cros-ec";
 		i2c-max-frequency = <100000>;
 		u-boot,i2c-offset-len = <0>;
+		u-boot,i2c-transaction-bytes = <3>;
 		ec-interrupt = <&gpx1 6 GPIO_ACTIVE_LOW>;
 	};
 };