samples: code_relocation_nocopy: Update configuration for nRF54H20 DK

It is no longer needed to specify the read command parameters in dts,
as the flash driver is capable of obtaining those from SFDP structures,
so the redundant properties are removed.

After commit 612fd940 got merged, both
the EXMIF and flash nodes in dts are not enabled by default so this
needs to be done in the overlay.

Although nrfutil is still not capable of programming the external
flash on nRF54H20 DK, thus `west flash` will not work in this case,
`nrf54h20dk/nrf54h20/cpuapp` is added as an allowed platform so that
the sample is at least built in CI for it.

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