net: dsa: bcm_sf2: support BCM4908's integrated switch

BCM4908 family SoCs come with integrated Starfighter 2 switch. Its
registers layout it a mix of BCM7278 and BCM7445. It has 5 integrated
PHYs and 8 ports. It also supports RGMII and SerDes.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210106213202.17459-3-zajec5@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>