dt-bindings: fttmr010: Add ast2600 compatible

The ast2600 contains a fttmr010 derivative.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191107094218.13210-5-joel@jms.id.au