[riscv] Inhibit SBI console when a serial console is active

[riscv] Inhibit SBI console when a serial console is active

When a native serial driver is enabled for the system console device
specified via "/chosen/stdout-path", it is very likely that this will
correspond to the same physical serial port used for the SBI debug
console.

Inhibit input and output via the SBI console whenever a serial console
is active, to avoid duplicated output characters and unpredictable
input behaviour.

Signed-off-by: Michael Brown <mcb30@ipxe.org>