sh: sh7264: Remove bogus SSU GPIO function definitions

SH7264 has no Synchronous Serial Communication Unit (SSU).
Remove the bogus enum IDs, which caused holes in pinmux_func_gpios[].

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191218194812.12741-6-geert+renesas@glider.be