clk: Fix build warnings

‘all_lists’ and ‘orphan_list’ is accessed only when DEBUG_FS is defined.
Thus, make their compilation conditional to fix the below warnings introduced
by commit 27b8d5f7 ("clk: flatten clk tree in debugfs"):
drivers/clk/clk.c:40:27: warning: ‘all_lists’ defined but not used [-Wunused-variable]
drivers/clk/clk.c:46:27: warning: ‘orphan_list’ defined but not used [-Wunused-variable]

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Cc: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>