ARM: shmobile: r8a73a4: add Z2 clock support

The Z2 clock on r8a73a4 is used to clock the 4 Cortex A7 cores on the SoC.
Add a definition for this clock to later use it from the arm_big_little
CPUFreq driver.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>