m68k: Rework BI_VIRT_RNG_SEED as BI_RNG_SEED

m68k: Rework BI_VIRT_RNG_SEED as BI_RNG_SEED

This is useful on !virt platforms for kexec, so change things from
BI_VIRT_RNG_SEED to be BI_RNG_SEED, and simply remove BI_VIRT_RNG_SEED
because it only ever lasted one release, and nothing is broken by not
having it. At the same time, keep a comment noting that it's been
removed, so that ID isn't reused. In addition, we previously documented
2-byte alignment, but 4-byte alignment is actually necessary, so update
that comment.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: a1ee38ab1a75 ("m68k: virt: Use RNG seed from bootinfo block")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20220927130835.1629806-2-Jason@zx2c4.com
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>