bcm47xx: add support for kernel 5.4
Ethernet, switch, LEDs, buttons, USB, sysupgrade & LuCI were
successfully tested on BCM4706.
WARNING: Hack for BCM4710 adding BCM4710_PROTECTED_FILL_TLB() to the
local_r4k_flush_cache_sigtramp() could not be ported. That function has
been dropped in:
commit
adcc81f148d7 ("MIPS: math-emu: Write-protect delay slot emulation pages")
commit
3315b6b336c8 ("MIPS: Delete unused flush_cache_sigtramp()")
it's unsure if that chipset will still work reliably.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>