[ARM] Add thread_notify infrastructure

Some machine classes need to allow VFP support to be built into the
kernel, but still allow the kernel to run even though VFP isn't
present.  Unfortunately, the kernel hard-codes VFP instructions
into the thread switch, which prevents this being run-time selectable.

Solve this by introducing a notifier which things such as VFP can
hook into to be informed of events which affect the VFP subsystem
(eg, creation and destruction of threads, switches between threads.)

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/iwmmxt.S

@@ -285,7 +285,7 @@ ENTRY(iwmmxt_task_switch)
 	bne	1f				@ yes: block them for next task
 
 	ldr	r5, =concan_owner
-	add	r6, r2, #(TI_IWMMXT_STATE - TI_CPU_DOMAIN) @ get next task Concan save area
+	add	r6, r2, #TI_IWMMXT_STATE	@ get next task Concan save area
 	ldr	r5, [r5]			@ get current Concan owner
 	teq	r5, r6				@ next task owns it?
 	movne	pc, lr				@ no: leave Concan disabled