vexpress/spc: 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.

Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>