i2c: imx: change register offset representation

the I2C register offset may different between SoCs,
to provid support for all these chips, split the
register offset into a fixed base address and a
variable shift value, then the full register offset
will be calculated by
reg_off = ( reg_base_addr << reg_shift)

Signed-off-by: Jingchang Lu <b35083@freescale.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>