irqchip: renesas-irqc: Add functional clock to bindings

The external IRQ controller has a functional clock, which is used for
power management. Document it.

Fix a typo in the r8a73a4 SoC name while we're at it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lkml.kernel.org/r/1426704961-27322-4-git-send-email-geert+renesas@glider.be
Signed-off-by: Jason Cooper <jason@lakedaemon.net>