ARM: shmobile: r7s72100: Add MTU2 device to DT

Add the MTU2 counter to the r7s72100 device tree and make it disabled by
default.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
[horms+renesas@verge.net.au correct irq number]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>