pia-am335x: update defaults for firststart special variables

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

- mux I2C1 bus if neccessary for EEPROM programming
- handle special case without EEPROM on PM module
- only try to write the expansion EEPROM if a board name was defined

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