tpm: Refactor TIS and add a SPI attachment

Summary:
Though mostly used in x86 devices, TPM can be used on others, with a
direct SPI attachment. Refactor the TPM 2.0 driver set to use an
attachment interface, and implement a SPI bus interface.

Test Plan:
Tested on a Raspberry Pi 4, with a GeeekPi TPM2.0 module (SLB9670
TPM) using security/tpm2-tools tpm2_getcaps for very light testing against the
spibus attachment.

Reviewed by: kd
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D45069