ARM: pxa: fix pxa_ssp_* declarations

The functions declared in include/linux/pxa2xx_ssp.h are
defined in plat-pxa/ssp.c, which can also be built for
PLAT_MMP, but may be disabled there. This can lead to
both unresolved symbols at link time and to duplicate
symbols at compile time for random configurations.

Changing the #ifdef in the header file to match the
Kconfig symbol that decides if the file is built solves
both problems.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Daniel Mack <zonque@gmail.com>