phy: phy-brcm-usb-init: Some Low Speed keyboards fail on 7271

Enable the the Low Speed Keep Alive signal on the 7271b0 by setting
the LS_KEEP_ALIVE bit in the USB CTRL OBRIDGE register otherwise
some Dell Low Speed keyboards fail.

Also do a little cleanup of the EBRIDGE ESTOP_SCB_REQ bit. Since
this is only used on one platform, remove it from the platform
tables and just use "if (family == ").

Fixes: 49859e55 ("phy: usb: phy-brcm-usb: Add Broadcom STB USB phy driver")
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>