powerpc/mpc512x: fix sparce warnings for non static symbols

Fix warnings:
symbol 'clockctl' was not declared. Should it be static?
symbol 'rate_clks' was not declared. Should it be static?
symbol 'dev_clks' was not declared. Should it be static?
symbol 'mpc5121_clk_init' was not declared. Should it be static?

Signed-off-by: Anatolij Gustschin <agust@denx.de>