arm: aspeed: Add clock-names property to timer node

The merging of a number of clocksource drivers into fttmr010 means we
require clock-names to be specified in the Aspeed timer node, else the
clocksource fails to probe and boot hangs.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>