drivers: tpm: atmel_twi: implement get_desc operation

Without get_desc, the tpm command will not provide a
description of the device in 'tpm device' or 'tpm info'.

Due to the characteristics of the Atmel TPM it isn't
possible to determine certain attributes (e.g open/close
status) without using the TPM stack (compare Infineon
and ST TPM drivers), so just print out the chip model
and udevice name as the identifier.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>