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