ARM: pxa: allow selecting just one of TREO680/CENTRO

There are two variants of the palmtreo machine, the 680 and the
centro, and Kconfig allows selecting one or both of them.
This changes the board file so that it's actually possible
to build all configurations that are allowed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>