wlcore: spi: add wl18xx support

Add support for using with both wl12xx and wl18xx.

- all wilink family needs special init command for entering wspi mode.
  extra clock cycles should be sent after the spi init command while the
  cs pin is high.
- Use inverted chip select for sending a dummy 4 bytes command that
  completes the init stage.

Signed-off-by: Eyal Reizer <eyalr@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>