ppc4xx_i2c: Rewrite to model hardware more closely

Rewrite to make it closer to how real device works so that guest OS
drivers can access I2C devices. Previously this was only a hack to
allow U-Boot to get past accessing SPD EEPROMs but to support other
I2C devices and allow guests to access them we need to model real
device more properly.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>