HID: i2c-hid: support regulator power on/off

On some boards, we need to enable a regulator before using the HID, and
it's also nice to save power in suspend by disabling it. Support an
optional "vdd-supply" and a companion initialization delay.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: linux-input@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>