drivers: pcie_ep: iproc: ensure config and api are const

The `config` and `api` members of `struct device` are expected
to be `const`. This also improves reliability, as `config`
and `api` are stored in rom rather than ram, which has the
potential to be corrupted at runtime in the absense of an MMU.

Signed-off-by: Chris Friedt <cfriedt@meta.com>