drivers: Add basic Atmel SAM USART driver

Added basic USART driver for Atmel SAM MCU family. USART is a very
comprehensive module, currently only basic features sufficient to
support printf functionality are implemented by this driver.

Tested on Atmel SMART SAM E70 Xplained board

Origin: Original

Jira: ZEP-978
Change-Id: Ia9710e4069243fb6c30de45953dfc1fe1266b63a
Signed-off-by: Piotr Mienkowski <Piotr.Mienkowski@schmid-telecom.ch>