tty: serial: cpm_uart: remove unused cpm_uart_nr declaration

cpm_uart_nr has been removed since
commit 0b2a2e5b ("cpm_uart: Remove !CONFIG_PPC_CPM_NEW_BINDING
code"), so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Link: https://lore.kernel.org/r/20220911093654.3222701-1-cuigaosheng1@huawei.com


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