samples: usb: hid-keyboard: enable and use remote wakeup on button press

USB remote wakeup capability is a common feature of keyboards,
allowing them to wake up a suspended host. Currently there is no USB sample
that demonstrates remote wakeup, which makes basic evaluation of the
functionality non-trivial.

Modify the hid-keyboard sample with a minimalistic solution
of demonstrating remote wakeup capability. While it is not required to have
been configured prior to suspend to issue a wakeup request, from usability
perspective I'd say it makes sense.

Signed-off-by: Benedek Kupper <kupper.benedek@gmail.com>