FRV: Use asm/generic-hardirq.h

Use asm/generic-hardirq.h to build asm/hardirq.h and also remove the unused
idle_timestamp field in irq_cpustat whilst we're at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>