pinctrl: sh-pfc: Mark run-time debug code __init

All run-time debug code is called from sh_pfc_init(), which is __init.

Fixes: 6161b39a

 ("pinctrl: sh-pfc: Validate pinmux tables at runtime when debugging")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>