ARM: shmobile: Restrict SCU support to SoCs that have it

Currently support for the ARM Cortex-A9 Snoop Control Unit is included
unconditionally, while only Renesas multicore Cortex-A9 SoCs have this
kind of SCU.

This decreases kernel image size by ca. 300 bytes on SoCs without such
an SCU.

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