i2c: sh_mobile: remove platform_data

No platform currently upstream makes use of this platform_data anymore.
The ones that did are converted to DT meanwhile. So, remove it. The old
platforms likely don't have the 'clks_per_cnt' feature, otherwise it
would have been implemented by now. And in the unlikely case they need
to setup a different bus speed, we should rather go for a generic i2c
platform data just for that.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>