cortex_m: poll S_REGRDY on register r/w

cortex_m: poll S_REGRDY on register r/w

Accordingly to arm documentation [1], chapter C1.6.4, the
operation to read/write from/to core registers can require time,
and the specific flag DHCSR.S_REGRDY has to be polled to verify
that the operation has been completed.
The lack of check on S_REGRDY causes OpenOCD to fail handling
correctly the core registers on a Cortex-M4 emulated in a slow
FPGA, and it could also fail on devices clocked at very low speed
while using a fast adapter.

Poll S_REGRDY as specified in [1] while either reading or writing
the core registers.
A timeout of 0.5s is added. This could still be too small in some
extremely slow cases, but at least now we log the timeout event,
which can help tracking down such odd issue.
During register read include in the polling loop the read of DCRSR
and to flush the JTAG queue only once.
During register write, relax the write in DCRSR by removing the
atomicity that is now useless since followed by the atomic read to
S_REGRDY.
During register read include the read of DCRSR inside the polling
loop to relax the read of S_REGRDY since followed by the atomic
read to DCRSR.

This change has the drawback of adding other transfers to the
adapter while reading/writing the registers, so it is expected to
introduce some speed degradation during step-by-step.

[1] DDI0403E - "ARMv7-M Architecture Reference Manual"

Change-Id: I61f454248f11a3bec6dcf4c58a50c5c996d7ef81
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/5319
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>