drivers: serial: Add MSP-EXP432P401R UART

This patch adds support for on board UART_0 on MSP-EXP432P401R-LAUNCHXL.
Driver makes use of driverlib available in ROM by default, thus saving
code space.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>