lib: utils: Add Xilinx uartlite implementation

[chenguokai]

Uartlite is a device commonly used on Xilinx platforms. We add OpenSBI support for uartlite so that OpenSBI boots on SoCs that utilize uartlite for output.

[avpatel]

We have mailing list based patch review so it would be great if you can send this patch to OpenSBI mailing list.

You need to join OpenSBI mailing list using following link http://lists.infradead.org/mailman/listinfo/opensbi

Make sure you use "git send-email" to send the patches.

Regards, Anup