Docs: Update required Mbed TLS version to 2.7.9

Also adds a missing include in tfm_mbedtls_config.h that was causing
issues with some later versions of Mbed TLS.

Change-Id: If504f97470c74a03c7ea49687b242c826fa1f4de
Signed-off-by: Jamie Fox <jamie.fox@arm.com>
diff --git a/platform/ext/common/tfm_mbedtls_config.h b/platform/ext/common/tfm_mbedtls_config.h
index 61f6652..d5aeeea 100644
--- a/platform/ext/common/tfm_mbedtls_config.h
+++ b/platform/ext/common/tfm_mbedtls_config.h
@@ -2679,4 +2679,6 @@
 #include MBEDTLS_USER_CONFIG_FILE
 #endif
 
+#include "mbedtls/check_config.h"
+
 #endif /* MBEDTLS_CONFIG_H */