[platform/target][warnings] fix -Wmissing-declarations warnings in platform/ and target/

Mostly driver code in various platforms. There are still some warnings
in this part of the tree in lesser-used platforms.

platform/include/platform/timer.h

@@ -26,10 +26,8 @@ typedef enum handler_return (*platform_timer_callback)(void *arg, lk_time_t now)
 status_t platform_set_periodic_timer(platform_timer_callback callback, void *arg, lk_time_t interval);
 
 /* If the platform implements a full dynamic (can be set to arbitary points in the future) timer */
-#if PLATFORM_HAS_DYNAMIC_TIMER
 status_t platform_set_oneshot_timer (platform_timer_callback callback, void *arg, lk_time_t interval);
 void     platform_stop_timer(void);
-#endif
 
 __END_CDECLS