clk: versatile: Remove CLK_IS_ROOT

This flag is a no-op now. Remove usage of the flag.

Cc: Pawel Moll <pawel.moll@arm.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>