wl18xx: implement hw op to read PG version

Read the HW PG version of the 18xx chip from FUSE.

Based on an earlier patch by Luciano Coelho <coelho@ti.com>.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>