powerpc/xmon: Select CONSOLE_POLL for the 8xx

Powerpc 8xx requires CONSOLE_POLL to get udbg_putc() and
udbg_getc() in CPM uart driver.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3d10a274516e9be8c4b0dc679a2840cdc1588872.1608716197.git.christophe.leroy@csgroup.eu