coresight: etm4x: Fix handling of ETM management registers access via sysfs

If ETM management registers are accessed directly, device may break
down due to the trace core is powered down. so modify the function
from coresight_etm4x_simple_func to coresight_etm4x_cross_read to
ensure the trace core is powered up before the register is accessed.

Change-Id: I935c08385bcca763f5bdc34b9ccd9ff27bbb1af0
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>