Documentation: devices.txt: Fix minors for ttyCPM*

ttyCPM* devices belong to CPM_UART driver at the first place
and that driver provides 6 ports.

Fixes: e29c3f81 ("Documentation: devices.txt: reconcile serial/ucc_uart minor numers")
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/27d7124cf86157e2a27c2b039e769041994d3f22.1691992627.git.christophe.leroy@csgroup.eu


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