NIU clocks are supplying the interconnect connections to specific
peripherals and are currently not controlled in any way.
So to prevent things falling apart at strange moments, mark all
niu clocks as critical.

Most of them where marked as CLK_IGNORE_UNUSED, but that doesn't
help if a parent clock then gets disabled.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>