clk: mvebu: Add the xtal clock for Armada 3700 SoC

This clock is the parent of all the Armada 3700 clocks. It is a fixed
rate clock which depends on the gpio configuration read when resetting
the SoC.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>