tty: serial: Prepare cleanup of powerpc's asm/prom.h

powerpc's asm/prom.h brings some headers that it doesn't
need itself.

In order to clean it up, first add missing headers in
users of asm/prom.h

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/49fc0d4b6446da630b1e9f29c4bab38f8ed087bf.1648833419.git.christophe.leroy@csgroup.eu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>