wl12xx: move partition table definition to io.c

wl12xx: move partition table definition to io.c

Up till now we only needed to access the partition table in boot.c.
But to add support for reading the MAC address from the FUSE in
testmode, we will have to change the partition in testmode.c.

Thus, we move the partition table to io.c and export it via io.h.  It
makes more sense to have it in the io part anyway.

Signed-off-by: Luciano Coelho <coelho@ti.com>