clocksource/drivers/fttmr010: Fix aspeed-2500 initialization

The recent changes made the fttmr010 to be more generic and support different
timers with a very few differences like moxart or aspeed.

The aspeed timer uses a countdown and there is a test against the aspeed2400
compatible string to set a flag.

With the previous patch, we added the aspeed2500 compatible string but without
taking care of setting the countdown flag.

Fix this by specifiying a init function and pass the aspeed flag to a common
init function.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>