xtensa: ISS: fix rs_put_char

ISS serial console prints garbage instead of symbols printed via
rs_put_char. gcc optimizes away putting prined symbol into memory buffer
because there's no evidence that the buffer is used afterwards.

Make rs_put_char and rs_write use simc_write that has explicit wmb.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>