proc: unify PROC_DEVICETREE config

Microblaze and PPC both use PROC_DEVICETREE, and OLPC will as well.. put
the Kconfig option into fs/ rather than in arch/*/Kconfig.

Signed-off-by: Andres Salomon <dilinger@queued.net>
[grant.likely@secretlab.ca: changed depends to PROC_FS && !SPARC]
[grant.likely@secretlab.ca: moved to drivers/of/Kconfig]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>