bcm53xx: add testing support for kernel 5.10

bcm53xx: add testing support for kernel 5.10

It still requires fixing PCIe support:
[    6.644699] pcie_iproc_bcma bcma0:7: host bridge /axi@18000000/pcie@12000 ranges:
[    6.652217] pcie_iproc_bcma bcma0:7:   No bus range found for /axi@18000000/pcie@12000, using [bus 00-ff]
[    6.661833] OF: /axi@18000000/pcie@12000: Missing device_type
[    6.667622] pcie_iproc_bcma: probe of bcma0:7 failed with error -12
[    6.673985] pcie_iproc_bcma bcma0:8: host bridge /axi@18000000/pcie@13000 ranges:
[    6.681514] pcie_iproc_bcma bcma0:8:   No bus range found for /axi@18000000/pcie@13000, using [bus 00-ff]
[    6.691137] pcie_iproc_bcma: probe of bcma0:8 failed with error -12
[    6.697522] pcie_iproc_bcma bcma0:9: host bridge /axi@18000000/pcie@14000 ranges:
[    6.705048] pcie_iproc_bcma bcma0:9:   No bus range found for /axi@18000000/pcie@14000, using [bus 00-ff]
[    6.714669] pcie_iproc_bcma: probe of bcma0:9 failed with error -12

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>