arm64: dts: medaitek: mt8395-nio-12l: Set i2c6 pins to bias-disable

GPIOs 25 and 26 do not support pull-up/pull-down when those are muxed
as I2C6's SDA6/SCL6 lines: set those to bias-disable to avoid warning
messages from the pinctrl driver.

Fixes: 96564b1e ("arm64: dts: mediatek: Introduce the MT8395 Radxa NIO 12L board")
Link: https://lore.kernel.org/r/20240409114211.310462-3-angelogioacchino.delregno@collabora.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>