arm64: enable CMT/TMU support for Renesas SoC

Renesas R-Car gen3 SoCs have both CMT and TMU timers, so we have to enable
building them in Kconfig.platforms (as they don't normally have the prompts
in Kconfig).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>