boards: b_u585i_iot02a: Add note on disabling TZ

Adds a note on how to disable TrustZone on the board, to be able to
reflash non TrustZone samples. Without this step, flashing will fail
for non TF-M or TrustZone sample applications.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>