Kernel/Devices: Introduce the Device Control Device

This device will assist userspace to manage hotplug events.
A userspace application reads a DeviceEvent entry until the return value
is zero which indicates no events that are queued and waiting for
processing.
Trying to read with a buffer smaller than sizeof(DeviceEvent) results in
EOVERFLOW.
For now, there's no ioctl mechanism for this device but in the future an
acknowledgement mechanism can be implemented via ioctl(2) interface.

Kernel/API/DeviceEvent.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+struct DeviceEvent {
+    int state;
+    int is_block_device;
+    unsigned major_number;
+    unsigned minor_number;
+
+    enum State {
+        Removed = 0x01,
+        Inserted = 0x02,
+        Recovered = 0x03,
+        FatalError = 0x04,
+    };
+};