Add instructions to erase PS on Musca S1

[LDong-Arm]

Since TF-M v1.3.0, the Protected Storage (PS) on Musca S1 is in its QSPI flash. If tests fail due to existing data in the PS, it should be erased before a rerun.

The erasing of the PS on Musca S1 cannot be automated because

• TF-M and Mbed OS are located in the target's MRAM, but the target's DAPLink only allows writing an image to QSPI when the BOOT switch is at QSPI. This prevents us from using one universal hex image that programs both MRAM and QSPI.
• The QSPI flash is not accessible by the non-secure application (e.g. Mbed OS) when the TF-M secure binary is using it - the control register is inaccessible. We cannot do the erase programmatically.

This commits adds an eraser image, musca_s1_ps_erase.hex, and a troubleshooting section in README.md explaining how to do erase.

[LDong-Arm]

Unfortunately we don't have an automated way to erase QSPI flash in CI, apart from the following.

Without the commit Platform: Enabled QSPI flash on Musca_S1 to store PS content in TF-M, the QSPI flash can be accessed from Mbed OS without problems. If the storage goes wrong on a CI board, at least we can flash & run an application based on an older TF-M (e.g. v1.2) to do the erase. This does not require changing the BOOT switch which is impossible on a CI board. It's still manual maintenance though.

Maybe another option is use a bare metal application without TF-M? Though our current code base in Mbed OS has no such support for Musca S1.