fsi: scom: Constify scom_ids

The only usage of scom_ids is to assign its address to the id_table
field in the fsi_driver struct, which is a const pointer, so make it
const to allow the compiler to put it in read-only memory

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>