tpm/ppi: Constify static struct attribute_group

The only usage of ppi_attr_grp is to put its address in an array of
pointers to const struct attribute_group. Make it const to allow the
compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>