ramips: enable I2C_CHARDEV

Expose I2C busses with a chardev device. This is required to control the
PSE controller on the Ubiquiti UniFi Flex Switch.

Signed-off-by: David Bauer <[email protected]>

target/linux/ramips/mt7621/config-5.4

@@ -100,6 +100,7 @@ CONFIG_HIGHMEM=y
 CONFIG_HZ_PERIODIC=y
 CONFIG_I2C=y
 CONFIG_I2C_BOARDINFO=y
+CONFIG_I2C_CHARDEV=y
 CONFIG_I2C_GPIO=y
 CONFIG_I2C_MT7621=y
 CONFIG_INITRAMFS_SOURCE=""