ARM: dts: socfpga: Add unit name to clock nodes

Most clock nodes in Arria5, Cyclone5 and Arria10 have a reg property but
does not have a unit name. This will trigger several warnings like this
one (when compiled with W=1):

Node /soc/clkmgr@ffd04000/clocks/periph_pll has a reg or ranges
property, but no unit name

Add the corresponding unit name to each node.

Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>