pia-am335x: switch to AUTOBOOT_KEYD

- Microchips USB-UART converters produce some undefined output after
  power-on, probably caused by autobauding checks. This would prevent
  the boards from booting, when a debug cable is plugged in.
- AUTOBOOT_KEYD allows to specify the keys that must be pressed to abort
  the normal boot process.

NOTE: You must use the small 's' key to abort booting now. There is no
      output on the console.

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