clk: mvebu: staticize of_cpu_clk_setup

This symbol is used only in this file. The patch fix the following
sparse warning:
warning: symbol 'of_cpu_clk_setup' was not declared. Should it be static?

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>