MIPS: Add debugfs file to print the segmentation control registers

Add a new mips/segments debugfs file to print the 6 segmentation
control registers for supported cores. A sample from a proAptiv core
is given below:

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6137/


Segment   Virtual    Size   Access Mode   Physical   Caching   EU
-------   -------    ----   -----------   --------   -------   --
   0      e0000000   512M      MK           UND         U       0
   1      c0000000   512M      MSK          UND         U       0
   2      a0000000   512M      UK           000         2       0
   3      80000000   512M      UK           000         3       0
   4      40000000    1G       MUSK         UND         U       1
   5      00000000    1G       MUSK         UND         U       1

Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>