bootcount: Remove legacy I2C driver

The legacy I2C bootcounter will hereby be removed and eventually
be replaced by a driver model implementation in the follow-up commit.

The legacy driver has the following drawbacks:
	- It's not adhering to the driver model
	- Settings are grabbed from Kconfig rather than device tree
	- i2c_{read,write} are being used instead of dm_i2c_{read,write}

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