boards: nxp/mimxrt685_evk: add pinctrl for I3C block

This adds the pinctrl/pinmux device tree node for the I3C
block on RT685 SoC.

Note the the drive strengths and slew rates for SCL and SDA
are recommended by the NXP application note AN12796: RT600 I3C
Simple Master, which can downloaded from the NXP website.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>