fix(spm): fill TX buffer again after undelegate

Similarly to the fix done in [1], the RME undelegate operation on TX
buffer scrubs its contents in test_ffa_memory_share_fragmented_tx_realm,
so re-init the TX buffer again with the fragment after undelegate.

[1] https://review.trustedfirmware.org/c/TF-A/tf-a-tests/+/30027



Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I6a716d42f79474b8d884826a0a09eac01e487176