tpm: MAINTAINERS: Add myself as tpm maintainer

Since I'm actively maintaining the tpm subsystem for a few months now,
it's time to step up and be an official maintainer for the tpm subsystem,
atleast until I hear something different from my company.

The maintaining is done solely in my private time, out of private interest.
Speaking only on behalf of myself, trying to be as vendor neutral as possible.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>