clk: mvebu: add helper file for Armada AP and CP clocks

Clock drivers for Armada AP and Armada CP use the same function to
generate unique clock name. A third drivers is coming with the same
need, so it's time to move this function in a common file.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lkml.kernel.org/r/20190710134346.30239-3-gregory.clement@bootlin.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>