m68k: virt: Use RNG seed from bootinfo block

Other virt VMs can pass RNG seeds via the "rng-seed" device tree
property or via UEFI, but m68k doesn't have either. Instead it has its
own bootinfo protocol. So this commit adds support for receiving a RNG
seed from it, which will be used at the earliest possible time in boot,
just like device tree.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20220626111509.330159-1-Jason@zx2c4.com


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>