drivers: serial: implement stm32 uart async api

Add initial implementation of the uart async api
for stm32 mcus. This uses the dma controller
in normal mode for reception. In addition, to detect
reception of bytes we enable the idle line detection
interrupt.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>
Signed-off-by: Jun Li <jun.r.li@intel.com>
Signed-off-by: Giancarlo Stasi <giancarlo.stasi.co@gmail.com>