drivers: misc: grove_lcd_rgb: use i2c_dt_spec

Modernize the driver a little bit by using i2c_dt_spec. Note that the
RGB on its own is another I2C device connected to the same bus. Ideally,
both should be defined in Devicetree, but this has not been done for
now.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>