Enable FPGA-based SPI testing on Silicon Labs targets

[stevew817]

Description

This PR enables the SPI tests based on the new FPGA testing shield to pass on Silicon Labs targets by:

• Implementing spi_free(...) (which somehow got added to the HAL API and never implemented on our targets)
• Returning no hardware CS pins for an SPI master. Hardware CS is not supported in our HAL in the case of an SPI master, and this is due to legacy, since the official way of handling CS is with a DigitalOut in the application. Adding in support for hardware CS on SPI master now might break legacy applications.

Pull request type

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

Reviewers

Release Notes

[ciarmcom]

@stevew817, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[evedon]

CI started

[mbed-ci]

Test run: SUCCESS

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