tpm: of: Handle IBM,vtpm20 case when getting log parameters

A vTPM 2.0 is identified by 'IBM,vtpm20' in the 'compatible' node in
the device tree. Handle it in the same way as 'IBM,vtpm'.

The vTPM 2.0's log is written in little endian format so that for this
aspect we can rely on existing code.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Acked-by: Nayna Jain <nayna@linux.ibm.com>
Tested-by: Nayna Jain <nayna@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>