drivers/spi/tpm: make tpm_info object local to compilation unit

The tpm_info object is a global, but its symbol does not need to
be exposed to the world as its only used within tpm.c.

BUG=b:36598499

Change-Id: Idded3dad8d0d1c3535bddfb359009210d3439703
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19057
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)

src/drivers/spi/tpm/tpm.c

@@ -52,7 +52,7 @@ static struct tpm_spi_if tpm_if = {
 };
 
 /* Cached TPM device identification. */
-struct tpm2_info tpm_info;
+static struct tpm2_info tpm_info;
 
 /*
  * TODO(vbendeb): make CONFIG_DEBUG_TPM an int to allow different level of