coresight: Update the logic for obtaining and enabling reg_clk

If the relevant property is not configured, of_property_count_strings()
will return a negative value, so the function of_coresight_get_reg_clk()
should directly return nul. In the function coresight_enable_reg_clk,
if reg_clk is NULL, it means that no reg_clk needs to be
enabled, the function should directly return 0.

Change-Id: I4f195b7c4701353fe61591f28e336f8bce55e43d
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>