HID: rmi: introduce RMI driver for Synaptics touchpads

This driver add support for RMI4 over USB or I2C.
The current state is that it uses its own RMI4 implementation, but once
RMI4 is merged upstream, the driver will be a transport driver for the
RMI4 library.

Part of this driver should be considered as temporary. Most of the RMI4
processing and input handling will be deleted at some point.

I based my work on Andrew's regarding its port of RMI4 over HID (see
https://github.com/mightybigcar/synaptics-rmi4/tree/rmihid )
This repo presents how the driver may looks like at the end:
https://github.com/mightybigcar/synaptics-rmi4/blob/rmihid/drivers/input/rmi4/rmi_hid.c

Without this temporary solution, the workaround we gave to users
is to disable i2c-hid, which leads to disabling the touchscreen on the
XPS 11 and 12 (Haswell generation).

Related bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=1048314
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1218973

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/Kconfig

@@ -657,6 +657,14 @@ config HID_SUNPLUS
 	---help---
 	Support for Sunplus wireless desktop.
 
+config HID_RMI
+	tristate "Synaptics RMI4 device support"
+	depends on HID
+	---help---
+	Support for Synaptics RMI4 touchpads.
+	Say Y here if you have a Synaptics RMI4 touchpads over i2c-hid or usbhid
+	and want support for its special functionalities.
+
 config HID_GREENASIA
 	tristate "GreenAsia (Product ID 0x12) game controller support"
 	depends on HID