phy: rockchip: usbdp: Drop rockchip_u3phy_uboot_init()

Remove the rockchip_u3phy_uboot_init() function, it has no caller and is
not needed with proper driver model use.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>