Update submodule public_regulated_data_types to latest Fri Apr 2 12:3…

…9:07 UTC 2021

    - public_regulated_data_types in PX4/Firmware (9c6b014): https://github/commit/7f5489e6e916cc8b13db0582dcf9930e225594b9
    - public_regulated_data_types current upstream: https://github/commit/53a7dbbf85554e11517c13ad91b9efb871599ad1
    - Changes: https://github/compare/7f5489e6e916cc8b13db0582dcf9930e225594b9...53a7dbbf85554e11517c13ad91b9efb871599ad1

    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)