dts: arm: atmel: Enable user-nrst by default

The datasheet for both sam4s/same70 specifies that the NRST pin
is an input after reset, used as a user reset.

Add the user-nrst property to match the default.

Fixes #43306

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>