tty: serial: cpm_uart: Add udbg support for enabling xmon

In order to use xmon with powerpc 8xx, the serial driver
must provide udbg_putc() and udpb_getc().

Provide them via cpm_put_poll_char() and cpm_get_poll_char().

This requires CONFIG_CONSOLE_POLL.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/e4471bf81089252470efb3eed735d71a5b32adbd.1608716197.git.christophe.leroy@csgroup.eu


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