ARM: shmobile: sh73a0 dtsi: Add Cortex-A9 TWD node

Add a node for the Private Timer and Watchdog, as found in the Cortex-A9
MPCore.

Without this, there's no clocksource available during early kernel
initialization, before cmt1 is initialized, leading to a lock-up if
CONFIG_CPU_IDLE=y.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>