pxa: move serial driver to drivers/serial

cpu/pxa/Makefile

drivers/serial/Makefile

drivers/serial/serial_pxa.c

include/configs/cerf250.h

include/configs/cradle.h

include/configs/csb226.h

include/configs/delta.h

include/configs/innokom.h

include/configs/logodl.h

include/configs/lubbock.h

include/configs/pleb2.h

include/configs/pxa255_idp.h

include/configs/trizepsiv.h

include/configs/wepep250.h

include/configs/xaeniax.h

include/configs/xm250.h

include/configs/xsengine.h

include/configs/zylonite.h