ktime.h: Introduce ktime_ms_delta

This patch adds a reusable time difference function which returns the
difference in millisecond, as often used in some driver code, e.g.
mtd/test, media/rc, etc.

Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Acked-by: Arnd Bergmann <arnd@linaro.org>
Cc: zhang.lyra@gmail.com
Cc: davem@davemloft.net
Cc: john.stultz@linaro.org
Cc: dborkman@redhat.com
Link: http://lkml.kernel.org/r/1418793095-18780-1-git-send-email-zhang.chunyan@linaro.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>