tty: serial: Drop unused efm32 serial driver

Support for this machine was just removed, so drop the now unused UART
driver, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210115155130.185010-7-u.kleine-koenig@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>