samples: boards: nrfx: Do not require (d)ppi node to be enabled

The sample needs to check in Kconfig whether PPI or DPPI is present
on the target platform in order to enable the proper allocator in nrfx.
It should not use the `HAS_HW_NRF_*` symbols for this as those take
into account also the status of the corresponding DT node, not only
its presence.

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