i2c: i801: Use MMIO if available

Newer versions of supported chips support MMIO in addition to legacy
PMIO register access. Probe the MMIO PCI BAR and use faster MMIO
register access if available.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/b4748b7a-aac5-445c-b813-20c4d2c23ec3@gmail.com