leds: trigger: Introduce a MTD (NAND/NOR) trigger

This commit introduces a MTD trigger for flash (NAND/NOR) device
activity. The implementation is copied from IDE disk.

This trigger deprecates the "nand-disk" LED trigger, but for backwards
compatibility, we still keep the "nand-disk" trigger around.

The motivation for deprecating the "nand-disk" LED trigger is that
it only works for NAND drivers, whereas the "mtd" LED trigger
is more generic (in fact, "nand-disk" currently only works for
certain NAND drivers).

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>