ARM: dts: r8a7792: Add missing clock for secondary CA15 CPU core

Currently only the primary CPU in the CA15 cluster has a clocks
property, while the secondary CPU core is driven by the same clock.
Add the missing clocks property to fix this.

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