firmware: raspberrypi: Report the fw variant during probe

The driver already reported the firmware build date during probe.
The mailbox calls have been extended to also report the variant
 1 = standard start.elf
 2 = start_x.elf (includes camera stack)
 3 = start_db.elf (includes assert logging)
 4 = start_cd.elf (cutdown version for smallest memory footprint).
Log the variant during probe.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>

firmware: raspberrypi: Report the fw git hash during probe

The firmware can now report the git hash from which it was built
via the mailbox, so report it during probe.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>