tpm_tis: Add compatible string atmel,at97sc3204

Commit 420d4398 ("tpm_tis: Allow tpm_tis to be bound using DT")
added the fallback compatible "tcg,tpm-tis-mmio" to the TPM TIS driver,
but not the chip-specific "atmel,at97sc3204".  However it did document
it as a valid compatible string.

Add it to tis_of_platform_match[] for consistency.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>