openocd: remove CamelCase symbols *xPSR*

We have left the camelcase symbols *xPSR* for some time, to avoid
any conflict with possibly pending patches in gerrit.
With the approaching v0.12.0-rc1, it's time to revisit it.
The patches in gerrit that conflict with this rename are all not
merge-able due to conflicts or due to negative review.

Drop these CamelCase symbols.

Change-Id: Ifbac4c1df9cc55994e024971a2aaebeed2ea4ed3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7155
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>