boards: nrf54h20dk: Make execution from RAM default for PPR

In practice, PPR is intended to be running code from RAM, so make this
the default choice for the `nrf54h20dk/nrf54h20/cpuppr` board target.
Keep the MRAM execution option as a `xip` variant of that target,
replacing the `ram` one.

Align the default `cpuapp` configuration for copying PPR's image to RAM
before it boots the child processor.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>