soc/mediatek/mt8173: Remove cast of NULL* to void *

`NULL` already has that type in coreboot.

```
src/include/stddef.h:#define NULL ((void *)0)
```

Change-Id: I73aeaef178be8779020c436732952aa732e90c46
Reported-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/22296
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/soc/mediatek/mt8173/bl31_plat_params.c

@@ -16,5 +16,5 @@
 
 void *soc_get_bl31_plat_params(bl31_params_t *bl31_params)
 {
-	return (void *)NULL;
+	return NULL;
 }