drivers: Add Atmel SAM serial (UART) driver

Added serial (UART) driver for Atmel SAM MCU family.

Note:
- Error handling is not implemented
- The driver works only in polling mode, interrupt mode is
  not implemented.

Tested on Atmel SMART SAM E70 Xplained board

Origin: Original

Jira: ZEP-1959
Change-Id: I3e770fd1feb2ddf92cf405a9aa17be92eb32e19b
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>