rockchip: pinctrl: rk3399: add support for I2C8

The RK3399 has a total of 9 I2C controllers.  To support these, the
enum in periph.h is extended and the mapping from the IRQ numbers to
the peripheral-ids is extended to ensure that pinctrl requests are
passed through to the function configuring the I2C pins.

For I2C8, the pinctrl is implemented and tested (on a RK3399-Q7) using
communication with the FAN53555 connected on I2C8.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>