ARM: cache_v7: use __weak

This is not only more readable but also prevents a warning
about a missing prototype. The prototypes which are actually
missing are added.

cc: Albert Aribaud <albert.u.boot@aribaud.net>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Reviewed-by: Tom Rini <trini@ti.com>