pm: add new 19.0.0 commands

This is functionally correct, but I have no idea what these are meant to represent.
These functions are completely unused on NX.

stratosphere/pm/source/pm_boot_mode_service.cpp

@@ -22,6 +22,7 @@ namespace ams::pm {
 
         /* Global bootmode. */
         constinit BootMode g_boot_mode = BootMode::Normal;
+        constinit u32 g_unknown = 0;
 
     }
 
@@ -47,4 +48,14 @@ namespace ams::pm {
         pm::bm::SetMaintenanceBoot();
     }
 
+    void BootModeService::GetUnknown(sf::Out<u32> out) {
+        out.SetValue(g_unknown);
+    }
+
+    Result BootModeService::SetUnknown(u32 val) {
+        R_UNLESS(val <= 3, pm::ResultUnknown7());
+        g_unknown = val;
+        R_SUCCEED();
+    }
+
 }