linux: CONFIG_I2C_BRCMSTB=y to improve HDMI I2C

This patch enables the CONFIG_I2C_BRCMSTB Linux kernel option, to
ensure proper support for the `brcm,bcm2711-hdmi-i2c` device node.

Without this configuration, the I2C bus associated with HDMI on
Broadcom BCM2711-based RPi4/400 systems may fail to probe, causing
issues with HDMI communication and display initialization.

See also:

- https://git.yoctoproject.org/meta-raspberrypi/commit/?id=aea47ad612bcfbfe07fb941694d3012e018607c0

linux_additional.defconfig

@@ -5,3 +5,4 @@ CONFIG_NETDEVICES=y
 CONFIG_CAN_GS_USB=y
 CONFIG_USB_PRINTER=y
 CONFIG_USB_TMC=y
+CONFIG_I2C_BRCMSTB=y