Add overlay for SLB9760 Iridium /LetsTrust TPM

Device Tree overlay for the Infineon SLB9670 Trusted Platform Module add-on
boards, which can be used as a secure key storage and hwrng.
available as "Iridium SLB9670" by Infineon and "LetsTrust TPM" by
pi3g.

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