clocksource: armada-370-xp: Get reference fixed-clock by name

The Armada XP timer has two mandatory clock inputs: nbclk and refclk,
as specified by the device-tree binding.

This commit fixes the clock selection. Instead of hard-coding the clock
rate for the 25 MHz reference fixed-clock, obtain the clock by its name.

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: Gregory CLEMENT <gregory.clement@free-electrons.com>