coresight: fix handling of ETM management register access via sysfs

This patch add a coresight unit specific function coresight_cross_read
which can be used for ETM registers by providing a ETM specific read
function which does smp cross call to ensure the unit is powered up
before the register is accessed.

Change-Id: I4037028a171c8ca733513e82c4443b6e332a088c
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>