drivers: modem: interface: introduce UART interface driver layer

The UART-based modem interface layer implements the modem context
interface for Zephyr's UART APIs.  This driver closely resembles
the existing modem receiver, but conforming to the modem interface
agreements.

Example modem driver setup code looks like this:

/* create modem context object */
static struct modem_context mctx;

/* create uart interface data object and buffers */
static struct modem_iface_uart_data iface_data;
static u8_t iface_isr_buf[MDM_RECV_BUF_SIZE];
static u8_t iface_rb_buf[MDM_MAX_DATA_LENGTH];

iface_data.isr_buf = &iface_isr_buf[0];
iface_data.isr_buf_len = sizeof(iface_isr_buf);
iface_data.rx_rb_buf = &iface_rb_buf[0];
iface_data.rx_rb_buf_len = sizeof(iface_rb_buf);
ret = modem_iface_uart_init(&mctx.iface, &iface_data,
			    UART_DEV_NAME);

Signed-off-by: Michael Scott <mike@foundries.io>