pia-am335x-em: fix uart3/4 pinmux

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
pia-am335x-em: add support for GSM_STATUS input

- shows ON/OFF status of the GSM module

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>