defconfig: Enable I2C_MUX_PINCTRL in all Pi configs

To be used to switch between the camera/display pins, and
the HAT pins for i2c-0.
(arm64/bcmrpi3_defconfig was missing all the I2C mux support,
so enable it all)

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