Home-Box: do not enable pull-ups for hardware revision detection pins

Trying to enable these pull-ups is nonsense as the i.MX28 has no
pull-ups for this pins. There are only "pad keepers" but with
converse meaning: setting these bits _disables_ the keepers.

Signed-off-by: Michael Heimpold <mhe@iequalize.de>
Revert "Home-Box: enable internal pull-up on debug uart RX pin"

This reverts commit 84cca164353a493ae5a2aaa9ebb1d1e79208c86b.

Trying to enable a not existing pull-up on the i.MX28 is nonsense.
In fact, setting this bit disables the pad keepers and results
in collecting garbage from the boards own output.

Signed-off-by: Michael Heimpold <mhe@iequalize.de>