arm64: aplmca: constify _hw_if struct

[Klemens Nanni]

The only driver with a non-const struct.
Seen while looking at it for audio(9).

OK?

diff --git a/sys/arch/arm64/dev/aplmca.c b/sys/arch/arm64/dev/aplmca.c
index 07a53103be4..03ade86fee7 100644
--- a/sys/arch/arm64/dev/aplmca.c
+++ b/sys/arch/arm64/dev/aplmca.c
@@ -135,7 +135,7 @@ int aplmca_trigger_input(void *, void *, void *, int,
 int    aplmca_halt_output(void *);
 int    aplmca_halt_input(void *);
 
-struct audio_hw_if aplmca_hw_if = {
+const struct audio_hw_if aplmca_hw_if = {
        .set_params = aplmca_set_params,
        .get_props = aplmca_get_props,
        .allocm = aplmca_allocm,