[ARM] pxa: simplify ifdef in ssp.h

Try to keep minimum ifdef in ssp.h.  Some of the definitions are valid
only to specific SoCs, and device drivers should be responsible to take
care of these difference.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>