powerpc/512x: clk: 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: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>