ARM: pxa: make zylonite_pxa*_init declaration match code

When support for zylonite is disabled, the zylonite_pxa300_init
and zylonite_pxa320_init functions are not there, but the declaration
is still kept around if any other boards for the same soc are
enabled. This changes the declaration to be conditional on the
same symbol as the code.

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