Add softraid(4) RAID 1C boot support

Add softraid(4) RAID 1C boot support

This is the arm64 adaptation of stsp's (commitid: p55cmsVoEH0fRg77)
"add support for booting from RAID 1C softraid(4) volumes on amd64".

Tell the boot loader to decrypt 1C like C volumes and check the number of
disks in 1C like in 1C volumes -- no new code rquired.

Tested on SolidRun CEX7
OK stsp

NB:  While kernel and boot loader support root on softraid on arm64,
installboot(8) does not, i.e. default installations still require manual
disk preparation for now.