ppc4xx: Cleanup PPC4xx I2C infrastructure

This patch cleans up the PPC4xx I2C intrastructure:

- Use C struct to describe the I2C registers instead of defines
- Coding style cleanup (braces, whitespace, comments, line length)
- Extract common code from i2c_read() and i2c_write()
- Remove unneeded IIC defines from ppc405.h & ppc440.h

Signed-off-by: Stefan Roese <sr@denx.de>