Update submodule public_regulated_data_types to latest Thu Apr 1 13:0…

…1:56 UTC 2021

    - public_regulated_data_types in PX4/Firmware (8b3e0f1): OpenCyphal/public_regulated_data_types@7f5489e
    - public_regulated_data_types current upstream: OpenCyphal/public_regulated_data_types@53a7dbb
    - Changes: OpenCyphal/public_regulated_data_types@7f5489e...53a7dbb

    53a7dbb 2021-03-31 Pavel Kirienko - Change wording in uavcan.pnp.NodeIDAllocationData to reflect the less static nature of UAVCAN v1 (#111)
c3aa82f 2021-03-23 Pavel Kirienko - Update Readiness as discussed at the DS-015 call
fcc1062 2021-03-16 Pavel Kirienko - Doc amendment for uavcan.register.Access: mapping between registers and environment variables (#109)
342f358 2021-02-22 Pavel Kirienko - Remove misleading comment in battery.Status.0.2
1baa9cb 2021-02-22 Pavel Kirienko - Add link to Nunaweb and synchronize the description with the front page (#108)