clk: renesas: rcar-gen2: Remove superfluous CLK_RENESAS_DIV6 selects

CLK_RENESAS_CPG_MSSR selects CLK_RENESAS_DIV6, and CLK_RCAR_GEN2_CPG
selects CLK_RENESAS_CPG_MSSR, so there is no longer a need for the
individual R-Car Gen2 clock driver options to select CLK_RENESAS_DIV6.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200427193446.29738-1-geert+renesas@glider.be