clk: mvebu: add Armada XP SoC-centric clock init

This is moving core clock and clock gating init for Armada XP to
its own file and adds a Kconfig option. Also init functions are added
and declared so they get called on of_clk_init.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>