i2c: piix4: Move i2c_piix4 macros and structures to common header

Add a separate header file to relocate the common code from the i2c_piix4
driver, allowing the AMD ASF driver to utilize the same code.

Update the MAINTAINERS file to include information about the new common
header file.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>