bootcount: Add driver model I2C driver

This adds a generic I2C bootcounter adhering to driver model to replace
the previously removed legacy implementation.

There is no change in functionality, it can be used on any I2C device.
The device tree configuration may look like this for example:

	bootcount {
		compatible = "u-boot,bootcount-i2c";
		i2cbcdev = <&i2c_rtc>;
		offset = <0x11>;
	};

Signed-off-by: Philip Richard Oberfichtner <pro@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>