ARM: pxa: enable PXA310/PXA320 for DT-only build

ARM: pxa: enable PXA310/PXA320 for DT-only build

After commit b5aaaa666a85 ("ARM: pxa: add Kconfig dependencies for
ATAGS based boards"), the default PXA build no longer includes support
for the board files that are considered unused.

As a side-effect of this, the PXA310 and PXA320 support is not built
into the kernel any more, even though it should work in principle as
long as the symbols are enabled. As Robert points out, there are dts
files for zylonite and cm-x300, though those have not made it into the
mainline kernel.

Link: https://lore.kernel.org/linux-arm-kernel/m2sfglh02h.fsf@free.fr/
Reported-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>