clk: renesas: prepare for non R-Car clock drivers

Add new CONFIG_CLK_RCAR to control compilation of shared code for R-Car
clock drivers (renesas-cpg-mssr.c). Enable this for R-Car Gen2 and 3.

This is necessary so that CONFIG_CLK_RENESAS can be enabled, allowing
recursion into the drivers/clk/reneasas directory, without bringing in
the R-Car support code. The support code contains platform specific
access (TMU_BASE) which is not needed on other Renesas devices such as
RZ/N1.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>