northbridge/intel: add missing #include guards

I first found the missing of #include guards when I tried to include
both sandybridge/gma.h and sandybridge/sandybridge.h, but
sandybridge.h includes gma.h in it and gives a compile error.

Change-Id: I13fdb8014b82e6065be2064137b7ea10062deaca
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://review.coreboot.org/13775
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
16 files changed