ARM: spear: use multiplatform configuration options.

The spear platform is now multiplatform capable in principle,
and everything still builds when enabled. This slightly rearranges
the Kconfig options for spear to enable both single- and multiplatform
support. As a side-effect, even building the single spear kernel
can now enable spear3xx and spear6xx simultaneously, although
not together with spear13xx, because they are a different archicture
version (v7 instead of v5).

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>