acpi_create_mcfg_mmconfig: Zero-out the structure before filling.

Otherwise "reserved" fields end up with a garbage instead of predictable
value.

Change-Id: I8a036769a8f86f1d6752651601de2800f4f1bd00
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7014
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/arch/x86/boot/acpi.c

@@ -118,6 +118,7 @@ void acpi_add_table(acpi_rsdp_t *rsdp, void *table)
 int acpi_create_mcfg_mmconfig(acpi_mcfg_mmconfig_t *mmconfig, u32 base,
 				u16 seg_nr, u8 start, u8 end)
 {
+	memset(mmconfig, 0, sizeof(*mmconfig));
 	mmconfig->base_address = base;
 	mmconfig->base_reserved = 0;
 	mmconfig->pci_segment_group_number = seg_nr;