powerpc/powernv: Implement and use opal_flush_console

A new console flushing firmware API was introduced to replace event
polling loops, and implemented in opal-kmsg with affddff6
("powerpc/powernv: Add a kmsg_dumper that flushes console output on
panic"), to flush the console in the panic path.

The OPAL console driver has other situations where interrupts are off
and it needs to flush the console synchronously. These still use a
polling loop.

So move the opal-kmsg flush code to opal_flush_console, and use the
new function in opal-kmsg and opal_put_chars.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>