bcm63xx_enet: add BQL support

Add Byte Queue Limits support to reduce/remove bufferbloat in
bcm63xx_enet.

Signed-off-by: Sieng Piaw Liew <liew.s.piaw@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>