scripts: runners: nrf: Default to soft reset for the nRF52 series

The Nordic nRF52 series have a peculiarity that is not shared with any
other Nordic families of SoCs: the reset pin can be reconfigured as a
regular GPIO. This has an unintended consequence: if the user has
reconfigured the pin in Devicetree to be a GPIO, `west flash` will
override that and configure the IC to use it as a reset pin, and the
firmware at boot won't be able to switch it back to GPIO, because that
requires a UICR erase. This behavior is very confusing to users, because
the GPIO does not work at all, since it is now just a reset line.

With this patch, `west flash` defaults to using soft reset instead of
pin reset for the nRF52 family of devices, to avoid overwriting the
reset pin configuration that the user includes in the image.

In order to be able to continue to use pin reset for users that so
desire it, a new option `--pinreset` is added that forces the use of pin
reset. The existing `--softreset` option is left exactly as it was, but
it is now applicable only to families other than the nRF52.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>