ARM: dts: r8a7794: Add Z2 clock

Add the Z2 clock (Cortex-A7 CPU core clock), which uses a fixed divider,
and link the first CPU node to it.

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