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>

include/asm-arm/u-boot-arm.h

@@ -62,4 +62,7 @@ void	reset_timer_masked	(void);
 ulong	get_timer_masked	(void);
 void	udelay_masked		(unsigned long usec);
 
+/* cpu/.../timer.c */
+int	timer_init		(void);
+
 #endif	/* _U_BOOT_ARM_H_ */