ntb_hw_switchtec: Make switchtec_ntb_init_req_id_table() more general

This is a prep patch in order to support the crosslink feature which
will require the driver to setup the requester ID table in another
partition as well as it's own. To aid this, create a helper function
which sets up the requester IDs from an array.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>