ARM: shmobile: r8a7779: Add TMU devices to DT

Add the TMU0, TMU1 and TMU2 counters to the r8a7779 device tree and make
them disabled by default.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>