drm: Add SPI connector type

tinydrm drivers announce DRM_MODE_CONNECTOR_VIRTUAL for its SPI drivers.
Add a SPI connector type to match the actual connector.

X will list the connector as Unknown:

X.Org X Server 1.19.2
Release Date: 2017-03-02
<...>
[ 53523.905] (II) modeset(0): Output Unknown19-1 has no monitor section
[ 53523.908] (II) modeset(0): EDID for output Unknown19-1
[ 53523.910] (II) modeset(0): Printing probed modes for output Unknown19-1
[ 53523.911] (II) modeset(0): Modeline "320x240"x0.0    0.00  320 320 320 320  240 240 240 240 (0.0 kHz eP)
[ 53523.911] (II) modeset(0): Output Unknown19-1 connected
[ 53523.912] (II) modeset(0): Using exact sizes for initial modes
[ 53523.912] (II) modeset(0): Output Unknown19-1 using initial mode 320x240 +0+0

The weston source shows that it will be listed as UNNAMED.

v2: Split patch in core and driver changes, expand commit message (Daniel)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: David Lechner <david@lechnology.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719155916.62465-2-noralf@tronnes.org