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

The clock consumer (CCU) of the CGU must be able to check if a CGU
base clock is really running since access to the CCU registers
requires a running base clock. 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. Since
certain clocks can be cascaded this must be added to all clock gates.

The hang would occur if the boot ROM or boot loader didn't setup and
enable the USB0 clock. Then when the clk framework tried to access
the CCU register it would hang the system.

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