dt-bindings: hwmon: ltc2978: add support for ltm4673

Add LTM4673 to supported devices of LTC2978. Unlike other LTM46xx
devices, LTM4673 is a μModule regulator that outputs four channels.

Signed-off-by: Cedric Encarnacion <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/devicetree/bindings/hwmon/lltc,ltc2978.yaml

···
       - lltc,ltc7880
       - lltc,ltm2987
       - lltc,ltm4664
+      - lltc,ltm4673
       - lltc,ltm4675
       - lltc,ltm4676
       - lltc,ltm4677
···
       * ltc7880 : vout0 - vout1
       * ltc3883 : vout0
       * ltm4664 : vout0 - vout1
+      * ltm4673 : vout0 - vout3
       * ltm4675, ltm4676, ltm4677, ltm4678 : vout0 - vout1
       * ltm4680, ltm4686 : vout0 - vout1
       * ltm4700 : vout0 - vout1