mb/emulation/qemu-q35: Enable option for TPM

mb/emulation/qemu-q35: Enable option for TPM

This enables the mainboard to use a TPM if it is selected in the
configuration.  By default this does nothing, but it allows the
TPM to be enabled and used with the CONFIG_USER_TPM2 Kconfig option.

Using a TPM with QEMU requires either a physical TPM backend or
the swtpm package with a socket:

  -chardev socket,id=swtpm,path=/tmp/swtpm/socket
  -tpmdev emulator,id=tpm0,chardev=swtpm
  -device tpm-tis,tpmdev=tpm0

Change-Id: I0d79a5a0f590c57998ababb660b52d9e3ed2d484
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39836
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>