mtd: kconfig: replace PPC_OF with PPC

The PPC_OF is a ppc specific option which is used to mean that the
firmware device tree access functions are available. Since all the
ppc platforms have a device tree, it is aways set to 'y' for ppc.
So it makes no sense to keep a such option in the current kernel.
Replace it with PPC.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>