Bluetooth: controller: openisa: Fix to use compiler memory barrier

Fix OpenISA port to include compiler memory barrier as in
commit 1a14f8b3 ("Bluetooth: controller: Use DMB instead
of DSB").

Relates to commit 1af2b91c ("Bluetooth: controller: Fix
Tx Buffer Overflow") and to commit ef2ece82 ("Bluetooth:
controller: openisa: Fix sanitycheck failures") and to
commit c2fc629d ("Bluetooth: controller: 255 byte AD
payload support").

Fixes #31937.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>