irqchip/renesas-irqc: Add r8a7795 INTC-EX DT documentation

For some reason the name of the external interrupt controller
has changed name with r8a7795, so use "intc-ex" instead of "irqc"
as r8a7795 compat string to follow the friendly documentation.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by:  <horms@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: jason@lakedaemon.net
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20150928094228.32552.83336.sendpatchset@little-apple


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>