cmake: Fix console could not work on board frdm_kl25z

The CMakeLists lost the config for uart_mcux_lpsci.c, which is the
console driver for board frdm_kl25z. So just add "uart_mcux_lpsci.c"
into drivers/serial/CMakeLists.txt to make it work.

Signed-off-by: Jason Wang <jason.yanping.wang@hotmail.com>