[POWERPC] adbhid: Enable KEY_FN key reporting

When a Fn key is used in combination with another key in ADB keyboards
it will generate a Fn event and then a second event that can be a
different key than pressed (Fn + F1 for instance can generate Fn +
brightness down if it's configured like that).  This enables the
reporting of the Fn key to the input system.

As Fn is a dead key for most purposes, it's useful to report it so
applications can make use of it.  One example is apple_mouse
(https://jake.ruivo.org/uinputd/trunk/apple_mouse/) that emulates the
second and third keys using a combination of keyboard keys and the mouse
button.  Other applications may use the KEY_FN as a modifier as well.
I've been updating and using this patch for months without problems.

Signed-off-by: Aristeu Rozanski <aris@ruivo.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>