serial: sh-sci: Drop the sci_fck clock fallback

All platforms that used to define an sci_fck clock have now switched to
the fck name. Remove the fallback code.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>