i2c: thunderx: Add i2c driver for ThunderX SOC

The ThunderX SOC uses the same i2c block as the Octeon SOC.
The main difference is that on ThunderX the device is a PCI device
so device probing is done via PCI, interrupts are MSI-X. The
clock rates can be set via device tree or ACPI.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>