ARM: pxa/mmp: remove traces of plat-pxa

The missing include directory caused a W=1 warning that can be
trivially fixed. I also noticed references in the marvell.rst
documentation that can be removed at the same time.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>