dt-bindings: clock: Remove gcc_npu_at_clk and gcc_npu_trig_clk

Update binding documentation to remove gcc_npu_at_clk
and gcc_npu_trig_clk since these clocks are managed
by the always-on processor (AOP).

Change-Id: Id77f8257707d513dd94fa1bac1517d11ee0145af
Signed-off-by: Vivek Aknurwar <viveka@codeaurora.org>