dt-bindings: input: iqs7222: Reduce 'linux,code' to optional

Following a recent refactor of the driver to properly drop unused
device nodes, the 'linux,code' property is now optional. This can
be useful for applications that define GPIO-mapped events that do
not correspond to any keycode.

Fixes: 44dc42d2 ("dt-bindings: input: Add bindings for Azoteq IQS7222A/B/C")
Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/Y1SROIrrC1LwX0Sd@nixie71


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>