hw/riscv: Enable TPM backends

Imply the TPM sysbus devices. This allows users to add TPM devices to
the RISC-V virt board.

This was tested by first creating an emulated TPM device:

    swtpm socket --tpm2 -t -d --tpmstate dir=/tmp/tpm \
        --ctrl type=unixio,path=swtpm-sock

Then launching QEMU with:

    -chardev socket,id=chrtpm,path=swtpm-sock \
    -tpmdev emulator,id=tpm0,chardev=chrtpm \
    -device tpm-tis-device,tpmdev=tpm0

The TPM device can be seen in the memory tree and the generated device
tree.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/942
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220427234146.1130752-7-alistair.francis@opensource.wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/riscv/virt.rst

@@ -162,3 +162,23 @@ The minimal QEMU commands to run U-Boot SPL are:
 To test 32-bit U-Boot images, switch to use qemu-riscv32_smode_defconfig and
 riscv32_spl_defconfig builds, and replace ``qemu-system-riscv64`` with
 ``qemu-system-riscv32`` in the command lines above to boot the 32-bit U-Boot.
+
+Enabling TPM
+------------
+
+A TPM device can be connected to the virt board by following the steps below.
+
+First launch the TPM emulator
+
+    swtpm socket --tpm2 -t -d --tpmstate dir=/tmp/tpm \
+        --ctrl type=unixio,path=swtpm-sock
+
+Then launch QEMU with:
+
+    ...
+    -chardev socket,id=chrtpm,path=swtpm-sock \
+    -tpmdev emulator,id=tpm0,chardev=chrtpm \
+    -device tpm-tis-device,tpmdev=tpm0
+
+The TPM device can be seen in the memory tree and the generated device
+tree and should be accessible from the guest software.