phy: phy-brcm-usb: fixup BCM4908 support

phy: phy-brcm-usb: fixup BCM4908 support

Just like every other family BCM4908 should get its own enum value. That
is required to properly handle it in chipset conditional code.

The real change is excluding BCM4908 from the PLL reprogramming code
(see brcmusb_usb3_pll_54mhz()). I'm not sure what's the BCM4908
reference clock frequency but:
1. BCM4908 custom driver from Broadcom's SDK doesn't reprogram PLL
2. Doing that in Linux driver stopped PHY handling some USB 3.0 devices

This change makes USB 3.0 PHY recognize e.g.:
1. 04e8:6860 - Samsung Electronics Co., Ltd Galaxy series, misc. (MTP mode)
2. 1058:259f - Western Digital My Passport 259F

Broadcom's STB SoCs come with a set of SUN_TOP_CTRL_* registers that
allow reading chip family and product ids. Such a block & register is
missing on BCM4908 so this commit introduces "compatible" string
specific binding.

Fixes: 4b402fa8e0b7 ("phy: phy-brcm-usb: support PHY on the BCM4908")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220218172459.10431-1-zajec5@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>