ARM: shmobile: kzm9g-reference dts: Add r2025sd rtc node

Add a DT node for the R2025D real-time clock, which is connected to
i2c0.

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