pru-software-support: bump to latest version 5.9.0

Update to the latest version of the PRU software support [1]. Compared
with v5.8.0, there aren't code or license changes, just some extra
release notes.

[1] https://git.ti.com/cgit/pru-software-support-package/pru-software-support-package



Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>