dt-bindings: display: panel: mipi-dbi-spi: Add io-supply

Add documentation for the new io-supply property, which specifies the
regulator for the I/O voltage supply on platforms where the panel
panel power and I/O supplies are separate.

Signed-off-by: Otto Pflüger <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Noralf Trønnes <[e-mail]>
Signed-off-by: Noralf Trønnes <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/panel/panel-mipi-dbi-spi.yaml

···
   The standard defines the following interface signals for type C:
   - Power:
     - Vdd: Power supply for display module
+      Called power-supply in this binding.
     - Vddi: Logic level supply for interface signals
-    Combined into one in this binding called: power-supply
+      Called io-supply in this binding.
   - Interface:
     - CSx: Chip select
     - SCL: Serial clock
···
     description: |
       Controller data/command selection (D/CX) in 4-line SPI mode.
       If not set, the controller is in 3-line SPI mode.
+
+  io-supply:
+    description: |
+      Logic level supply for interface signals (Vddi).
+      No need to set if this is the same as power-supply.
 
 required:
   - compatible