wlcore/wl12xx: add plt_init op and move the code to wl12xx

PLT mode needs to be initialized differently for each chip.  This
patch adds an operation to init PLT and moves the existing PLT
initialization into the wl12xx driver.

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