If the serial tx/rx are floating, it can happen that bogus characters
are detected on the line at boot time. This leads to U-boot accidentally
thinking someone pressed a key to stop autoboot, thus stopping booting process.
This can happen if the serial cable is not connected. There are hardware
pull-ups on the board connected to serial cable VBUS.
To solve this when the cable is not plugged, enable internal pull-ups as well
for the tx/rx lines.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>