clock-gcc-plutonium: Fix wrong debug_rpm_clk selection value

rpm_debug_clk needs to use a special value in gcc_debug_mux. Fix the
wrong value used in previous implementation.

Change-Id: Ia2949180c948a657c3805aa50f3a4f92ecf4e5ec
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>