[ARM] pxa/em-x270: prepare addition of eXeda machine to em-x270.c

Change several GPIO assignment from static to run-time
  Split MFP table to common and EM-X270 specific parts
  Introduce em_x270_module_init

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Eric Miao <eric.miao@marvell.com>