tile: force CONFIG_TILEGX if ARCH != tilepro

This allows configuring with allnoconfig (for tilegx) or allyesconfig
(for tilepro) without creating an unbuildable configuration.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>