Document support for booting from RAID 1C on sparc64

Document support for booting from RAID 1C on sparc64

The bootloader (ofwboot) got support some time ago, but installboot(8) had
to be fixed as well for multi-chunk volumes (1C requires at least two).

As of today, the latest sparc64 install72.img can perform a default install
onto a passphrase-protected 1C volume across four chunk disks, resulting in
a bootable system without requiring manual invervention, just like amd64.