armcm_irq: Move ARM Cortex-M irq handling to new file

The irq handling in sam3x8e isn't specific to the sam3x8e proccessor -
it's generic for all armcm type machines.  So, move the definitions
into a new file generic/armcm-irq.c

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

src/sam3x8e/timer.c

@@ -5,9 +5,9 @@
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
 #include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "board/irq.h" // irq_disable
 #include "board/misc.h" // timer_from_us
 #include "command.h" // shutdown
-#include "irq.h" // irq_disable
 #include "sam3x8e.h" // TC0
 #include "sched.h" // sched_timer_kick