serial: sh-sci: Remove the platform data dma slave rx/tx channel IDs

Only SH platforms still use platform data for the sh-sci, and none of
them declare DMA channels connected to the SCI. Remove the corresponding
platform data fields and simplify the driver accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>