clk: renesas: r8a7795: make SD clk definition specific for GEN3

About SD clocks: The clock type is Gen3 specific, the callbacks are all
Gen3 specific; I think the clock definition should also be Gen3 specific
and not in the general header file.

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