boards: nrf52840dk_nrf52840: Do not enable arduino_serial by default

This is a follow-up to commit 25d7a09a.

The arduino_serial/uart1 node should not be enabled by default because
even if an application does not use it, the default CONFIG_SERIAL=y
setting causes that it is anyway initialized, so the UARTE peripheral
acquires its assigned pins (and they cannot be used in other way) and
its enabled receiver causes increased current consumption (by ~500 uA)
when the CPU is sleeping. This affects e.g. the boards/nrf/system_off
sample.
Applications that actually need to use this UART or shields that use
the arduino_serial node should enable the node explicitly.

Keep this node disabled by default for the nrf52840dk_nrf52840 board
and also for boards whose definitions are mostly copies of the above:
nrf52833dk_nrf52833 and nrf21540dk_nrf52840.

Update also accordingly a few overlay files in tests/ that were
disabling this node because of the pins it undesirably acquired.

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