Merged in juniskane/nuttx_stm32l4/stm32_serial_patches_pr (pull request #504)

Stm32, stm32l4 serial patches

* stm32: serial: add interface to get uart_dev_t by USART number, stm32_serial_get_uart

* stm32: serial: do not stop processing input in SW flow-control mode

* stm32l4: serial: do not stop processing input in SW flow-control mode

* stm32l4: serial: suspend serial for Stop mode

Approved-by: Gregory Nutt <gnutt@nuttx.org>