dt-bindings: fttmr010: Add ast2600 compatible

The ast2600 contains a fttmr010 derivative.
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191107094218.13210-5-joel@jms.id.au

Documentation/devicetree/bindings/timer/faraday,fttmr010.txt

@@ -11,6 +11,7 @@ Required properties:
   "moxa,moxart-timer", "faraday,fttmr010"
   "aspeed,ast2400-timer"
   "aspeed,ast2500-timer"
+  "aspeed,ast2600-timer"
 
 - reg : Should contain registers location and length
 - interrupts : Should contain the three timer interrupts usually with