arch/tile: prefer "tilepro" as the name of the 32-bit architecture

With this change, you can (and should) build with ARCH=tilepro for the
current 32-bit chips.  Building with ARCH=tile continues to work, but
we've renamed the defconfig file to tilepro_defconfig for consistency.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>