xen/arm: vpl011: Add a pl011 uart DT node in the guest device tree

xen/arm: vpl011: Add a pl011 uart DT node in the guest device tree

The SBSA UART node format is as specified in
Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt and given below:

ARM SBSA defined generic UART
------------------------------
This UART uses a subset of the PL011 registers and consequently lives
in the PL011 driver. It's baudrate and other communication parameters
cannot be adjusted at runtime, so it lacks a clock specifier here.

Required properties:
- compatible: must be "arm,sbsa-uart"
- reg: exactly one register range
- interrupts: exactly one interrupt specifier
- current-speed: the (fixed) baud rate set by the firmware

Currently the baud rate of 115200 has been selected as a default value,
which is one of the valid baud rate settings. Higher baud rate was
selected since an emulated pl011 can support any valid baud rate without
any limitation of the hardware.

Signed-off-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>