drivers: serial: Kconfig.nrfx: Filter out options unsupported on nRF54H20

On nRF54H20, only the new shim can be used and the enhanced poll out
cannot be enabled since there is no DPPI support for this SoC yet.

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