boards: nrf: Enable pull-ups on UART pins in Nordic DKs

This is a follow-up to commit 3656ba5a.

Enable pull-up resistors on UART pins RXD and CTS in Nordic DK boards
where it is possible that these pins are not connected to anything on
the board and could generate problems if left floating (on some DKs
also the Interface MCU can be disconnected with the switch "nRF only",
hence pull-ups enabled also on uart0 for them).
Those pull-ups can still be disabled (by deleting the added properties)
if necessary to use a given board in some specific hardware setup.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>