clock-plutonium: Fix gcc_debug_mux registration location

gcc_debug_mux is part of debug clock controller, not global clock
controller. When GCC registers, the base pointer of gcc_debug_mux
might not be mapped yet. Remove it from msm_clocks_gcc_plutonium.
gcc_debug_mux is already correctly registered in debug controller.

Change-Id: If0d1c0ffc32e118677046f4664aa98fa31a33ef0
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>