drm/bridge: DRM_CROS_EC_ANX7688 should depend on I2C_CROS_EC_TUNNEL

The ChromeOS EC ANX7688 bridge is connected to a ChromeOS Embedded
Controller, and is accessed using I2C tunneling through the Embedded
Controller.  Hence add a dependency on I2C_CROS_EC_TUNNEL, to prevent
asking the user about this driver when configuring a kernel without
support for the ChromeOS EC tunnel I2C bus.

Fixes: 44602b10

 ("drm/bridge: Add ChromeOS EC ANX7688 bridge driver support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/d107d1840b83607baee8571cc5d88973fc32b519.1622015323.git.geert+renesas@glider.be