kernel: Add KERNEL_DCB (Data Center Bridging)

kernel: Add KERNEL_DCB (Data Center Bridging)

The kmod-mlxsw-spectrum driver activated CONFIG_DCB indirectly already
on all targets which are building this driver. All other DCB capable
driver did not activate their DCB support.

CONFIG_DCB increases the uncompressed kernel size by about 7.8KB.

CONFIG_DCB is only needed some data center Ethernet cards and not used
on normal routers. Activate it only on the x86_64 and the armsr_arm64
target which are used on normal servers or in VMs.

Link: https://github.com/openwrt/openwrt/pull/17672
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>