dt-bindings: phy: brcm, brcmstb-usb-phy: add BCM4908 binding

BCM4908 uses the same PHY and may require just a slightly different
programming.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210106205838.10964-2-zajec5@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>