drivers/intel/gma: Add EDID retrieving functions.

Change-Id: I64f2fcc5ad52d6a0188d02b28769001ada718c4f
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/5278
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)

src/drivers/intel/gma/edid.h

@@ -0,0 +1 @@
+void intel_gmbus_read_edid(u32 gmbus_mmio, u8 bus, u8 slave, u8 *edid, u32 edid_size);