Drop three-wire serial (TWS) support

This subsystem has not been converted to driver model, there is only one
driver and only one board that uses it. Drop it and its CONFIG option.

Also drop the rtc4543 RTC driver since it uses TWS.

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