Platform/Sgi: define the macro ENABLE_GOP

Define and use the global macro ENABLE_GOP to enable the use of the
Graphics Output Protocol (GOP). Enabling this macro allows GOP protocol
to be used for display on the HDLCD controller of the platform. This
macro is set to false by default for the all supported platforms.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>