phy: rockhip-usb: Remove CLK_IS_ROOT

This flag is a no-op now (see commit 47b0eeb3

 "clk: Deprecate
CLK_IS_ROOT", 2016-02-02) so remove it.

Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>