arm64: dts: renesas: gen3: use 400kHz for I2C DVFS bus

The PMIC and EEPROM can operate at 400kHz, so use this speed.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>