am35xpia: enable 5V before checking expansion boards

- some expansion boards are supplied by 5V instead of 3.3V, so we need
  to enable 5V before trying to read the EEPROM
- also change EN_VCC_5V_PER pull-up to pull-down, as there is an
  external pull-down present as well

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
Merge branch 'pia-master'
pia35x: update default command line

- enable ext4 for rootfs
- mount rootfs read-only to allow easier boottime fsck

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