[ARM] S3C64XX: sparse warnings in arch/arm/plat-s3c64xx/s3c6400-clock.c

Fix the following sparse warnings in s3c6400-clock.c:

39:12: warning: symbol 'clk_ext_xtal_mux' was not declared. Should it be static?
66:12: warning: symbol 'clk_fout_apll' was not declared. Should it be static?
81:19: warning: symbol 'clk_mout_apll' was not declared. Should it be static?
91:12: warning: symbol 'clk_fout_epll' was not declared. Should it be static?
106:19: warning: symbol 'clk_mout_epll' was not declared. Should it be static?
126:19: warning: symbol 'clk_mout_mpll' was not declared. Should it be static?
148:12: warning: symbol 'clk_dout_mpll' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>