sh-sci: Get register size from platform device

There is much SCI of SoC having within,
and the register size is also different in everyone.
So get from platform device.

Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Acked-by: by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>