ARM: shmobile: rcar-gen2: Remove unused rcar_gen2_read_mode_pins()

After
  1. commit 9f5ce39d ("ARM: shmobile: rcar-gen2: Obtain extal
     frequency from DT"),
  2. commit 80951f04 ("ARM: shmobile: rcar-gen2: Stop passing
     mode pins state to clock driver"),
  3. and handling of debug resource reset,
there are no more users of rcar_gen2_read_mode_pins() left.
Remove the function and its support definitions.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>