NS16550 UART

This UART driver supports many chip variants and is used in mamy SoCs.

Required properties:
- compatible: "ns16550" or "nvidia,tegra20-uart"
- reg: start address and size of registers
- reg-shift: shift value indicating register size: 0=byte, 1=16bit,2=32bit etc.
- clock-frequency: input clock frequency for the UART (used to calculate the
    baud rate divisor)