Platform/Sgi: Initialize additional uart controllers

The IO virtualization block on reference design platforms allow
connecting non-discoverable devices such as PL011 UART. On platforms
that support this, initialize the UART controller connected to the IO
virtualization block.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I70bd3f790f51fa86707b0d300b3a70168731a4ff