readme: Add support for SGI-575 platform in readme.rst

FVP_CSS_SGI-575 is the Fixed Virtual Platform on which the
Trusted-Firmware-A Tests are executed for SGI-575.

Change-Id: I606d316d251e69f5a88208b4edde132d29856b3d
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
1 file changed