Starfive VisionFive 2 Board Support

[its-valentinvp]

This PR adds Starfive VisionFive 2 Board support to Keystone and addresses #339.
During our case study on secure peripheral access from keystone enclaves, we (@f1bu and I) also added VF2 board support to Keystone to allow for testing some prototypes on real hardware. We now want to contribute our changes to Keystone. We oriented on this previous PR (#386) and tried to be consistent with our changes (same directory structures and similar names, etc.) for easier reviews.

It's important to note that as secure boot is not implemented yet, this board support should only be used for testing and not for production.

We tested the PR on two different VF2 boards and were able to run all examples successfully. One exception is the tests.ke example which constantly displays a "[runtime] non-handable interrupt ..." message after some of the tests pass. It is triggered for test-fib-bench on the rdcycle operation. This is because we use Linux 6.7, and since Linux 6.6, the rdcycle operation is a privileged instruction (see [1]), which means it is no longer available for userland. One fix for this is to replace rdcycle with rdtime. After that change, the tests.ke example runs successfully (this change is not included in this PR but this problem should be addressed in the future).

To build the SD image, simply run the following script:

#!/bin/sh
export KEYSTONE_PLATFORM=starfive/visionfive2
export KEYSTONE_BITS=64
export BUILDROOT_CONFIGFILE=riscv64_starfive_visionfive2_defconfig
export BUILDROOT_TARGET=all
make -j$(nproc)

The sd image is located in build-starfive/visionfive264/buildroot.build/images/sdcard.img. To flash the image to a sd card you can use make flash (see mkutils/plat/starfive/visionfive2/run.mk).

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3