nb/intel/sandybridge: Make the mainboard_early_init hook optional

This adds an empty weakly linked default. The rationale behind this
change is that without the callback some features might not work
but that the result is likely still able to boot, so it can be made
optional.

Change-Id: I62c8010aa81fc45d208e9293feb2f45b45f34a82
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36780
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/src/northbridge/intel/sandybridge/sandybridge.h b/src/northbridge/intel/sandybridge/sandybridge.h
index 31d4358..d667e36 100644
--- a/src/northbridge/intel/sandybridge/sandybridge.h
+++ b/src/northbridge/intel/sandybridge/sandybridge.h
@@ -213,6 +213,8 @@
 void early_init_dmi(void);
 
 void pch_enable_lpc(void);
+/* mainboard_early_init: Optional mainboard callback run after console init
+   but before raminit. */
 void mainboard_early_init(int s3resume);
 void mainboard_config_superio(void);
 int mainboard_should_reset_usb(int s3resume);