ARM: shmobile: only call rcar_gen2_clocks_init() if present

The RZ/G1M (r8a7743) uses the R-Car Gen2 core, but not the R-Car Gen2 clock
driver. This is a harbinger of a transition for R-Car Gen2 SoCs. As the
process to get all the required pieces in place is somewhat complex it
seems useful to try to disentangle dependencies where possible.

The approach here is to temporarily disable calling rcar_gen2_clocks_init()
if no R-Car Gen2 SoC are configured and thus the symbol will not be
present.

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