ARM: cache: declare set_section_dcache

We declare the set_section_dcache function globally in the cache header, for
later use by e.g. machine specific code.

Signed-off-by: Vincent Stehlé <v-stehle <at> ti.com>
Cc: Tom Rini <trini <at> ti.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>