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/at91rm9200/Makefile b/cpu/arm920t/at91rm9200/Makefile
index 161ca94..73aeeac 100644
--- a/cpu/arm920t/at91rm9200/Makefile
+++ b/cpu/arm920t/at91rm9200/Makefile
@@ -31,10 +31,10 @@
 COBJS	+= dm9161.o
 COBJS	+= ether.o
 COBJS	+= i2c.o
-COBJS	+= interrupts.o
 COBJS	+= lxt972.o
 COBJS	+= reset.o
 COBJS	+= spi.o
+COBJS	+= timer.o
 COBJS	+= usb.o
 
 SRCS	:= $(SOBJS:.o=.S) $(COBJS:.o=.c)