Support decoding mem32 memory in the rk pcie driver

This is needed with some dtb files.

While here use a switch statement as the two options are mutually
exclusive in any iteration of the loop.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35680

(cherry picked from commit 1c799a6f29cadc7616362941c279dd0693c24645)