pia-am335x: add mmc device detection for microsd and emmc

- try to boot from µSD card by default
- if the boot fails, try eMMC

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
pia-am335x: always run board late init

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