ramips: fix mt7621 boot on v4.9

v4.9 CM code has a few bugs on this HW. Disable the GCR register access
during boot. This caused a cpu stall.

Signed-off-by: John Crispin <john@phrozen.org>

target/linux/ramips/patches-4.9/0098-disable_cm.patch

+Index: linux-4.9.14/arch/mips/kernel/mips-cm.c
+===================================================================
+--- linux-4.9.14.orig/arch/mips/kernel/mips-cm.c
++++ linux-4.9.14/arch/mips/kernel/mips-cm.c
+@@ -239,6 +239,7 @@ int mips_cm_probe(void)
+ 
+ 	/* disable CM regions */
+ 	write_gcr_reg0_base(CM_GCR_REGn_BASE_BASEADDR_MSK);
++	/*
+ 	write_gcr_reg0_mask(CM_GCR_REGn_MASK_ADDRMASK_MSK);
+ 	write_gcr_reg1_base(CM_GCR_REGn_BASE_BASEADDR_MSK);
+ 	write_gcr_reg1_mask(CM_GCR_REGn_MASK_ADDRMASK_MSK);
+@@ -246,7 +247,7 @@ int mips_cm_probe(void)
+ 	write_gcr_reg2_mask(CM_GCR_REGn_MASK_ADDRMASK_MSK);
+ 	write_gcr_reg3_base(CM_GCR_REGn_BASE_BASEADDR_MSK);
+ 	write_gcr_reg3_mask(CM_GCR_REGn_MASK_ADDRMASK_MSK);
+-
++*/
+ 	/* probe for an L2-only sync region */
+ 	mips_cm_probe_l2sync();
+