arm: pia-am335x-em: don't init DISP LED as LED but generic GPIO

- also make sure the display is turned on

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: don't turn the driver of the debug rs485 off

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