apalis/colibri_t30: avoid uart input from floating pins

Avoid UART input from floating RX pins on UARTB and UARTC (Colibri T30)
and UARTB, UARTC and UARTD (Apalis T30).

Note: Floating pins may cause spurious break conditions potentially
interrupting U-Boot's autoboot.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>