mmc: Move tegra loopback disable option to be under tegra

This is a configuration option specific to the tegra controller.

Doing it this way makes it show up directly under the tegra controller
option, indented one level, as "Disable external clock loopback".

The way it is now, it shows up at the end of the controller list, not
indented, as if it's some kind of generic MMC configuration option.

Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Tom Warren <twarren@nvidia.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>