arm64: dts: r8a7796: salvator-x: Populate EXTALR

It can be used for the watchdog.

Based on similar work for r8a7795/salvator-x by Wolfram Sang.

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