mtd: Add a function to report when the MTD dev list has been updated

We need to parse mtdparts/mtids again everytime a device has been
added/removed from the MTD list, but there's currently no way to know
when such an update has been done.

Add an ->updated field to the idr struct that we set to true every time
a device is added/removed and expose a function returning the value
of this field and resetting it to false.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Heiko Schocher <hs@denx.de>