of: Put all CONFIG_OF dependencies into a Kconfig menu block

All of the options in drivers/of/Kconfig depend on CONFIG_OF.  Putting
all of them inside a menu block simplifies the dependency statements.
It also creates a logical group for adding user selectable OF options.

This patch also changes (PPC_OF || MICROBLAZE) statements to (!SPARC)
so that those options are available to other architectures (and in
fact the !SPARC conditions should probably be re-evalutated since the
code is more generic now)

This patch also moves the definition of CONFIG_DTC from arch/* to
drivers/of/Kconfig

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>