powerpc: mpic: make mpic_subsys const

Since commit d492cc25 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the mpic_subsys variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240212-bus_cleanup-powerpc2-v2-3-8441b3f77827@marliere.net

arch/powerpc/include/asm/mpic.h

@@ -336,7 +336,7 @@ struct mpic
 #endif
 };
 
-extern struct bus_type mpic_subsys;
+extern const struct bus_type mpic_subsys;
 
 /*
  * MPIC flags (passed to mpic_alloc)

arch/powerpc/sysdev/mpic.c

@@ -49,7 +49,7 @@
 #define DBG(fmt...)
 #endif
 
-struct bus_type mpic_subsys = {
+const struct bus_type mpic_subsys = {
 	.name = "mpic",
 	.dev_name = "mpic",
 };