[raspberry] Clean up temporary fixes for old kernel, bump kernel to 5.15.76

[ashthespy]

• Remove the firmware hacks put in place for the v1.5 revision on 5.4.83 kernels
• Update to 5.15.76

This is the current kernel version in the raspberrypi-kernel package (_1.20221104-1).

[ashthespy]

@volumio any luck with the test images/QA?

FWIW, current upstream kernel is 5.15.84 (_1.20230106) But perhaps it's better to move to the 6.1.y LTS kernel and avoid double QA?

[Darmur]

not necessary anymore, cleanup has been done