i2c: Make I2C_ATR invisible

I2C Address Translator (ATR) support is not a stand-alone driver, but a
library.  All of its users select I2C_ATR.  Hence there is no need for
the user to enable this symbol manually, except when compile-testing.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>