mtd: fix build error in m25p80.c

While building an x86 distro kernel, I hit the following:

Kernel: arch/x86/boot/bzImage is ready  (#7)
ERROR: "of_mtd_parse_partitions" [drivers/mtd/devices/m25p80.ko]
undefined!

of_mtd_parse_partitions is defined with MTD_OF_PARTS, and that's only
built on PPC and microblaze.  The code in question should be wrapped w/
a stricter #ifdef.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>