drivers: serial: nrf: Make pull-ups on UART pins configurable

This is a follow-up to commit 3656ba5a.

Do not enforce pull-up resistors to be enabled on RXD and CTS pins
in nRF UART drivers, as in certain hardware designs this may be
undesirable or may even make certain hardware not working.
Instead, provide devicetree properties that allow enabling of those
resistors when it is actually needed.

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