drivers: serial: nrfx_uarte: Allow use of legacy shim on RISCV cores

Legacy shim takes less flash so it should be a first choice on cores
with less code memory (like RISCV cores on nrf54h20). Adding new
instances support to the legacy shim.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
3 files changed