Add 'handheld' chassis type

The 'handheld' chassis type describes portable devices with integrated
controls. The term is actively used for gaming devices.

This differs from 'handset' (phones) and 'tablet' (touch-first devices).

Note: SMBIOS defines "Hand Held" (0x0B) but it was historically used for
PDAs and is mapped to 'handset' by systemd.

Signed-off-by: Alexey Andreyev <dev@aa13q.ru>

Author: a-andreyev

source/chapter3-devicenodes.rst

@@ -61,6 +61,7 @@ are descendants. The full path to the root node is ``/``.
                                                * ``"server"``
                                                * ``"all-in-one"``
                                                * ``"tablet"``
+                                               * ``"handheld"``
                                                * ``"handset"``
                                                * ``"watch"``
                                                * ``"embedded"``