ec/google/chromeec: Move if EC_GOOGLE_CHROMEEC to i2c_tunnel/Kconfig

ec/google/chromeec: Move if EC_GOOGLE_CHROMEEC to i2c_tunnel/Kconfig

This change moves `if EC_GOOGLE_CHROMEEC` from chromeec/Kconfig to
chromeec/i2c_tunnel/Kconfig. This is done to make it clear that the
Kconfig file in i2c_tunnel is sourced unconditionally, but the configs
in i2c_tunnel/Kconfig are conditionally defined based on the
evaluation of if condition.

This change addressed the feedback received on
https://review.coreboot.org/c/coreboot/+/40515/11/src/ec/google/chromeec/Kconfig#200.

Change-Id: I66cd91d6b1813ff6d0fb7be719e2da65ac6ac23b
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43040
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>