Are there any interfaces supplied by L4 to access TPM?
There is a prototype server, which gives you a /dev/tpm like interface. Unfortunately is this server not public until now.
Are there any trusted platform application on L4?
Yes, we have some and we are currently working in the OpenTC project to build some more.
You can also have a look at the EMSCB project: http://www.emscb.org
On the website you can download a demo CD which contains a very simple TPM client application (it shows you some basic information about the TPM like version number, public key, and you can get random numbers from the TPM). The source code is also available.
More applications on L4 are currently under development in the EMSCB project, and as Bernhard said, in the OpenTC project.
Kind regards, Marcel