arm: timer and interrupt init rework

actually the timer init use the interrupt_init as init callback
which make the interrupt and timer implementation difficult to follow

so now rename it as int timer_init(void) and use interrupt_init for interrupt

btw also remane the corresponding file to the functionnality implemented

as ixp arch implement two timer - one based on interrupt - so all the timer
related code is moved to timer.c

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/arm920t/imx/Makefile b/cpu/arm920t/imx/Makefile
index d3352deb..28945e2 100644
--- a/cpu/arm920t/imx/Makefile
+++ b/cpu/arm920t/imx/Makefile
@@ -25,7 +25,9 @@
 
 LIB	= $(obj)lib$(SOC).a
 
-COBJS	= generic.o interrupts.o speed.o
+COBJS	+= generic.o
+COBJS	+= speed.o
+COBJS	+= timer.o
 
 SRCS	:= $(SOBJS:.o=.S) $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(SOBJS) $(COBJS))