time: add weak annotation to timer_read_counter declaration

A weak annotation is needed in order to prevent link errors when
get_ticks is overridden. This fixes sandbox build.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>

lib/time.c

@@ -37,7 +37,7 @@ unsigned long notrace timer_read_counter(void)
 #endif
 }
 #else
-extern unsigned long timer_read_counter(void);
+extern unsigned long __weak timer_read_counter(void);
 #endif
 
 unsigned long long __weak notrace get_ticks(void)