arm_adi_v5: do not deactivate power domains while trying to clear sticky error

arm_adi_v5: do not deactivate power domains while trying to clear sticky error

At OpenOCD start-up the operation of clearing the sticky error in
CTRL/STAT register ignores the current value of the power domains
bits CDBGPWRUPREQ and CSYSPWRUPREQ in the same register and
incorrectly set them to zero.
This abrupt disable does not follow the requirement in IHI0031 to
wait for the acknowledgment of power disabled before continuing.
The power domains are then re-enabled immediately after; it is
possible that such short disable period has passed undetected or
has been tested only on devices that do not implement the power
domains.
Anyway, this sequence is incorrect and can generate unexpected
and hard-to-debug issues while OpenOCD attaches to a running
target that implements power domains.

Anticipate the initialization of dap->dp_ctrl_stat and use it
while clearing the sticky bit. This has the additional effect of
avoiding a power disable in the error recovery part of the
function dap_dp_read_atomic().
Keep the same sequence of read/write in dap_dp_init() to avoid
breaking the initialization of some problematic target.
Add comments to document these choices.

Change-Id: I8d6da788f2dd11909792b5d6b69bc90fbe4df25d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4677
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>