Extend TXT Buffer with read access

[Blebowski]

Allow read access from TXT Buffer via Memory registers.

Read access will be allowed only when in the TXT Buffer
is in Non-transient state.

Migrated from GitLab: https://gitlab.fel.cvut.cz/canbus/ctucanfd_ip_core/-/issues/484

[Blebowski]

Work in progress done in:
https://gitlab.fel.cvut.cz/canbus/ctucanfd_ip_core/-/commit/5617fabf9dbb8458a01fc88ea5c7b8871c5672d0