ARM: mvebu: Add clock properties to Armada XP timer node

With the addition of the Armada XP reference clock, we can now model
accurately the available clock inputs for the timer: namely, nbclk
and refclk. For each of this clock inputs we assign a name, for the
driver to select as appropriate.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>