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>

hw/riscv/Kconfig

@@ -34,6 +34,7 @@ config RISCV_VIRT
     imply PCI_DEVICES
     imply VIRTIO_VGA
     imply TEST_DEVICES
+    imply TPM_TIS_SYSBUS
     select RISCV_NUMA
     select GOLDFISH_RTC
     select MSI_NONBROKEN