dt-bindings: iio: hmc425a: add entry for LTC6373

The LTC6373 is a silicon, 3-bit Fully-Differential digital instrumentation
amplifier that supports the following programmable gains (Vout/Vin):
G = 0.25, 0.5, 1, 2, 4, 8, 16 + Shutdown.

Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Dumitru Ceclan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/amplifiers/adi,hmc425a.yaml

···
   HMC540S 1 dB LSB Silicon MMIC 4-Bit Digital Positive Control Attenuator, 0.1 - 8 GHz
     https://www.analog.com/media/en/technical-documentation/data-sheets/hmc540s.pdf
 
+  LTC6373 is a 3-Bit precision instrumentation amplifier with fully differential outputs
+    https://www.analog.com/media/en/technical-documentation/data-sheets/ltc6373.pdf
 
 properties:
   compatible:
···
       - adi,adrf5740
       - adi,hmc425a
       - adi,hmc540s
+      - adi,ltc6373
 
   vcc-supply: true
 
···
         ADRF5740  - 4 GPIO connected to D2-D5
         HMC540S   - 4 GPIO connected to V1-V4
         HMC425A   - 6 GPIO connected to V1-V6
+        LTC6373   - 3 GPIO connected to A0-A2
     minItems: 1
     maxItems: 6
 
···
         ctrl-gpios:
           minItems: 4
           maxItems: 4
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: adi,ltc6373
+    then:
+      properties:
+        ctrl-gpios:
+          minItems: 3
+          maxItems: 3
 
 required:
   - compatible