phy: phy-brcm-usb: support PHY on the BCM4908

BCM4908 seems to have slightly different registers but works when
programmed just like the STB one.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210106205838.10964-3-zajec5@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>