m68k: coldfire: make symbol m523x_clk_lookup static

Fix sparse warnings:

arch/m68k/coldfire/m523x.c:31:19: sparse: sparse: symbol 'm523x_clk_lookup' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Jingjin <wangjingjin1@huawei.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>