dt: Implement an I2C pinctrl mux for BSC0.

BSC0 serves either the HAT EEPROM pins on the 40pin connector,
or the display and camera on a board specific pairing of either
GPIO 28&29, or 44&45.

Use I2C_MUX_PINCTRL to allow exposing both pairs of pins as I2C
busses.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>