RealView: Allow XVGA (1024x768) CLCD resolution on the RealView boards

This patch adds another clcd_panel structure for the above resolution
and makes it the default for the PB1176 and PB11MPCore platforms.

Signed-off-by: Colin Tuckley <Colin.Tuckley@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>