service/pm: Tidy up functionality related to SystemBootMode

Just minor tidying of interfaces.

src/core/hle/service/pm/pm.h

@@ -9,7 +9,12 @@ class ServiceManager;
 }
 
 namespace Service::PM {
-enum class SystemBootMode : u32 { Normal = 0, Maintenance = 1 };
+
+enum class SystemBootMode {
+    Normal,
+    Maintenance,
+};
+
 /// Registers all PM services with the specified service manager.
 void InstallInterfaces(SM::ServiceManager& service_manager);