lib: utils: Fix fdt_mpxy_init() not returning error code

It seems that current implementation doesn't fail on fdt_mpxy_init(),
because platforms might not have any MPXY devices. In fact, if there are
no MPXY devices, fdt_driver_init_all() will return SBI_OK.

More importantly, if there is any MPXY device which fails the
initialization, OpenSBI must check the error code and stop the booting.
Thus, this commit adds the return value for fdt_mpxy_init().

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250430091007.3768180-1-alvinga@andestech.com
Signed-off-by: Anup Patel <anup@brainfault.org>

include/sbi_utils/mpxy/fdt_mpxy.h

@@ -15,11 +15,11 @@
 
 #ifdef CONFIG_FDT_MPXY
 
-void fdt_mpxy_init(const void *fdt);
+int fdt_mpxy_init(const void *fdt);
 
 #else
 
-static inline void fdt_mpxy_init(const void *fdt) { }
+static inline int fdt_mpxy_init(const void *fdt) { return 0; }
 
 #endif