clk: lpc18xx-ccu: fix potential system hang when disabling unused clocks

CCU branch clock register must only be accessed while the base
(parent) clock is running. Access with a disabled base clock
will cause the system to hang. Fix this issue by adding code
that check if the parent clock is running in the is_enabled
clk_ops callback.

This hang would occur when disabling unused clocks after AMBA
runtime pm had already disabled some of the clocks.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>