sandbox: Move CONFIG_TPM_TIS_SANDBOX to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/tpm/Kconfig b/drivers/tpm/Kconfig
index e69de29..f408b8a 100644
--- a/drivers/tpm/Kconfig
+++ b/drivers/tpm/Kconfig
@@ -0,0 +1,7 @@
+config TPM_TIS_SANDBOX
+	bool "Enable sandbox TPM driver"
+	help
+	  This driver emulates a TPM, providing access to base functions
+	  such as reading and writing TPM private data. This is enough to
+	  support Chrome OS verified boot. Extend functionality is not
+	  implemented.