serial: Add driver for CMSDK (Cortex-M System Design Kit) APB UART

Add driver for CMSDK APB UART ports. The driver implements RX and TX
functionalities. Data transmission is implemented using polling and
interrupts. The driver default configuration for the port is to use
8bit data transmission, 1 stop bit, no parity control.

The driver exposes a public uart driver API and allows to register up
to five UART devices: from 'UART_0' to 'UART_4'. The driver performs
the required pinmux, the clock control configuration is left to the
platform.

The driver has been verified to work with the Hello World sample
application on a Beetle Board.

Jira: ZEP-1245
Change-Id: I5baf78b7659aae2a574d8e66205e6fd5eb579133
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>