mips: cache: Make flush_cache() weak to enable overwrite

This patch adds __weak to flush_cache() in lib/cache.c. This makes it
possible to overwrite this function by a platforms specific version,
like done with the Octeon base port.

Signed-off-by: Stefan Roese <sr@denx.de>

arch/mips/lib/cache.c

@@ -107,7 +107,7 @@ static inline unsigned long scache_line_size(void)
 	}								\
 } while (0)
 
-void flush_cache(ulong start_addr, ulong size)
+void __weak flush_cache(ulong start_addr, ulong size)
 {
 	unsigned long ilsize = icache_line_size();
 	unsigned long dlsize = dcache_line_size();