tty: hvc: Don't enable the RISC-V SBI console by default

The new SBI console has the same problem as the old one: there's only
one shared backing hardware and no synchronization, so the two drivers
end up stepping on each other.  This was the same issue the old SBI-0.1
console drivers had, but that was disabled by default when SBI-0.1 was.

So just mark the new driver as nonportable.

Reported-by: Emil Renner Berthing <kernel@esmil.dk>
Fixes: 88ead68e

 ("tty: Add SBI debug console support to HVC SBI driver")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240214153429.16484-2-palmer@rivosinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>