PCIe: add API to get BAR region size

[andrewboie]

Is your enhancement proposal related to a problem? Please describe.
We have pcie_get_mbar() to get a BAR MMIO base address, but no API to also obtain the region size. This means that memory-mapping drivers needs to use hard-coded region sizes instead of just querying it.

Describe the solution you'd like
An API to obtain a BAR region size given a BDF and a BAR index.

Describe alternatives you've considered
Change pcie_get_mbar() to return both address and size instead of just address.

Additional context
This is more of an annoyance since driver code already "knows" the bounds since it's written against a datasheet and you can just #define it.

[dcpleung]

Done in #29818