Drop digital thermometer and thermostat (DTT) drivers

This subsystem is quite old. It has been replaced with a driver-model
version (UCLASS_THERMAL). Boards are free to convert to that if required,
but here is a removal patch that could be applied in the meantime.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>