clk: renesas: Do not build clk-div6 for R8A7792

R-Car V2H does not have "DIV6" programmable clocks, hence there is no
need to build clk-div6.o.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>