ramips: enable I2C_CHARDEV in mt7621/config-5.10

I2C_CHARDEV used to be enabled in mt7621/config-5.4. Enable it in the
5.10 config, as it's required for PoE control on Unifi Switch Flex.

Fixes: b4aad29 ("ramips: add support for kernel 5.10")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

target/linux/ramips/mt7621/config-5.10

@@ -96,6 +96,7 @@ CONFIG_HAS_IOPORT_MAP=y
 CONFIG_HIGHMEM=y
 CONFIG_I2C=y
 CONFIG_I2C_BOARDINFO=y
+CONFIG_I2C_CHARDEV=y
 CONFIG_I2C_GPIO=y
 CONFIG_I2C_MT7621=y
 CONFIG_INITRAMFS_SOURCE=""