ARM: shmobile: Enable CPUFREQ configuration

This changes of configuration is multiplatform build only for r8a7790/r8a7791.

Signed-off-by: Gaku Inami <gaku.inami.xw@bp.renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>