dt-bindings: aspeed-sgpio: Add ast2600 sgpio

AST2600 SoC has 2 SGPIO master interfaces one with 128 pins another one
with 80 pins. Add ast2600-sgpiom compatibles and update descriptions to
introduce the max number of available gpio pins that AST2600 supported.

Signed-off-by: Steven Lee <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Joel Stanley <[e-mail]>

Documentation/devicetree/bindings/gpio/aspeed,sgpio.yaml

···
   - Andrew Jeffery <[e-mail]>
 
 description:
-  This SGPIO controller is for ASPEED AST2500 SoC, it supports up to 80 full
-  featured Serial GPIOs. Each of the Serial GPIO pins can be programmed to
-  support the following options
+  This SGPIO controller is for ASPEED AST2400, AST2500 and AST2600 SoC,
+  AST2600 have two sgpio master one with 128 pins another one with 80 pins,
+  AST2500/AST2400 have one sgpio master with 80 pins. Each of the Serial
+  GPIO pins can be programmed to support the following options
   - Support interrupt option for each input port and various interrupt
     sensitivity option (level-high, level-low, edge-high, edge-low)
   - Support reset tolerance option for each output port
···
     enum:
       - aspeed,ast2400-sgpio
       - aspeed,ast2500-sgpio
+      - aspeed,ast2600-sgpiom
 
   reg:
     maxItems: 1