serial: cpm_uart: implement the cpm_uart_early_write() function for console poll

The cpm_uart_early_write() function which was used for console poll
isn't implemented in the cpm uart driver.

Implementing this function both fixes the build when CONFIG_CONSOLE_POLL
is set and allows kgdboc to work via the cpm uart.

Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Reviewed-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>