timer_irq: Rename generic/timer.c to generic/timer_irq.c

Rename the file to make it clear that the code is helper functions for
boards with irq based timers.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/timer.c

@@ -6,13 +6,11 @@
 
 #include "board/irq.h" // irq_disable
 #include "board/misc.h" // timer_read_time
+#include "board/timer_irq.h" // timer_dispatch_many
 #include "command.h" // DECL_SHUTDOWN
 #include "sam3x8e.h" // TC0
 #include "sched.h" // sched_timer_kick
 
-// From generic/timer.c
-extern void timer_dispatch_many(void);
-
 // IRQ handler
 void __visible
 TC0_Handler(void)