fbdev: sh_mipi_dsi: add set_dot_clock() for each platform

Dot clock of SH MIPI are depends on each platform board.
This patch adds set_dot_clock() function for it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>