clocksource: armada-370-xp: Add detailed clock requirements in devicetree binding

Specifies the required clock inputs for each supported compatible.
Armada 370 requires a single clock phandle, and Armada XP requires
two clock phandles with clock-names "nbclk" and "fixed".

Cc: devicetree@vger.kernel.org
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>