[MTD] mtdpart.c: Allow eraseblock size != power of 2

Don't assume eraseblock size is power of 2.
Dataflash can have aligned eraseblock size.

From: Peter Menzebach <pm-mtd@mw-itcon.de>
Acked-by: Artem B. Bityutskiy <dedekind@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>