Disable MPU support on NRF52x platforms

[donatieng]

Description

This PR disables MPU support on NRF52x devices. Unfortunately, the Nordic Softdevice seems to attempt writes to addresses marked as "Write only" under certain conditions. For instance, this behaviour is triggered when attempting to discover GATT services on a remote peripheral, while in central mode.
Thanks to @pan- for the investigation.

MPU support can still be enabled if using Cordio, or if the Bluetooth feature is disabled.

This workaround creates an issue (MPU support disabled on NRF52x devices) - opening the following ticket to track it till the root cause is fixed and we can re-enable MPU on these platforms: #9181

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@ARMmbed/mbed-os-hal

[bulislaw]

We should track, investigate and possibly fix all of the platforms with disabled MPU. Do we need a ticket per board?

[cmonr]

CI started.
An answer to @bulislaw's question should still be provided.

Personal opinion: One constantly updated tracking issue should be sufficient.

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[cmonr]

Personal opinion: One constantly updated tracking issue should be sufficient.

Counter argument: Seperate tickets/isuses may make tracking easier.

[pan-]

Thanks for submitting the PR; the cause of the issue was a bit surprising to say the least.

[cmonr]

Will merge once at least one linked issue is posted here for tracking purposes cough @bulislaw @donatieng @c1728p9 cough

[donatieng]

cough the issue has been here all along cough (PR description)
#9181
😉

[kjbracey]

cough the issue has been here all along cough (PR description)

I think the confusion is arising because there's no "fixes" issue indicated, so people are assuming that #9181 is what this is fixing...

[donatieng]

Unfortunately we cannot fix the underlying bug as is it present in the Nordic Softdevice; that being said applications not using the Softdevice (including Cordio Link Layer) are unaffected

[donatieng]

Good point @kjbracey-arm, I'll clarify this in the PR's description

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts