powerpc64: make radix with superpages default

[luporl]

Summary

As Radix MMU with superpages enabled is now stable, make it the
default choice on supported hardware (POWER9 and above), since its
performance is greater than that of HPT MMU.

Test Plan

Tested on POWER8 and POWER9 VMs and physical machines.

[jhibbits]

Looks fine to me.

[alfredo]

For some reason this breaks the boot with VGA console on Blackbird.
I'm using D30626 locally, but reverted it as well. With D30626 reverted and adding kernel parameter 'hw.ofwfb.physaddr=0x000600c280000000' the kernel hangs as well with no output.

Works:
'hw.ofwfb.physaddr=0x000600c280000000 radix_mmu=0 vm.pmap.superpages_enabled=0'
'hw.ofwfb.physaddr=0x000600c280000000 radix_mmu=0 vm.pmap.superpages_enabled=1'

Doesn't work:
'hw.ofwfb.physaddr=0x000600c280000000 radix_mmu=1 vm.pmap.superpages_enabled=0'
'hw.ofwfb.physaddr=0x000600c280000000 radix_mmu=1 vm.pmap.superpages_enabled=1'

It seems like superpages doesn't hurt, but something must be changed in ofwfb to work with radix_mmu

[luporl]

D31232 fixes OFWFB with radix. After it lands it should be safe to make radix default.

[alfredo]

I tested this patch with D31232 few weeks ago on the Blackbird and it looks good to me once it's approved.