Added spi

[Frederikwag]

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https://github.com/Infineon/.github/blob/master/CONTRIBUTING.md
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CONTRIBUTING.md also tells you what to expect in the PR process.

Description
The submodule "Arduino-Core-Test" in the repo "XMC-for-Arduino" linked to an older version. The version of "Arduino-Core-Test" was updated to the newest one, the different tests were executed and it was documented which tests are also working. The files where adapted accordingly.

Related Issue
It's not an issue, but tasks that still need to be done:

1. Interrupts: It exists an open ticket for the interrupt-problem. A sending interrupt is not received by an receiving pin. The existing test shows that 4/6 tests don't pass, which CAN be traced back to this. Therefore, the interrupt-functionality needs to be solved first. BUG-Ticket needs to be created!
2. Timing: the following test function is not passed: RUN_TEST_CASE(time_single_internal, testDelayMicroseconds)
   --> Expected Delay: 500000 microseconds; Actual Delay: 506483 microseconds; Tolerance: 100 microseconds (result: values not within delta 100)
3. IIC is not working for 1 board only: make FQBN=Infineon:xmc:XMC4700_Relax_Kit PORT=COM28 UNITY_PATH=\Unity test_wire_connected1_pingpong monitor --> There is an open ToDo

Context