soc/intel/xeon_sp: Lock down DMICTL

This is required for CBnT.

Change-Id: I290742c163f5f067c8d529ddca8e2d8572ab6e6a
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47449
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/soc/intel/xeon_sp/pch.c b/src/soc/intel/xeon_sp/pch.c
index 5427952..2b35223 100644
--- a/src/soc/intel/xeon_sp/pch.c
+++ b/src/soc/intel/xeon_sp/pch.c
@@ -39,6 +39,9 @@
 	reg32 = (0x3f << 18) | ACPI_BASE_ADDRESS | 1;
 	pcr_write32(PID_DMI, PCR_DMI_ACPIBA, reg32);
 	pcr_write32(PID_DMI, PCR_DMI_ACPIBDID, 0x23a8);
+
+	reg32 = pcr_read32(PID_DMI, PCR_DMI_DMICTL);
+	pcr_write32(PID_DMI, PCR_DMI_DMICTL, reg32 | PCR_DMI_DMICTL_SRLOCK);
 }
 
 void bootblock_pch_init(void)