ACPI / amba: 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.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Graeme Gregory <graeme.gregory@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>