ARM: mv78xx0: 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: Andrew Lunn <andrew@lunn.ch>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>