clk: si5{14,351,70}: Remove CLK_IS_ROOT

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

Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Cc: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>