RISC-V: Announce real and available memory at boot

Most architectures print their total (real) and available memory during
boot. Properly initialize the realmem global and print these messages.
Also print the physical memory chunks (behind a bootverbose flag).

Reviewed by: markj
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D20496