wlcore: move wl12xx_platform_data up and make it truly optional

The platform data is used not only by wlcore-based drivers, but also
by wl1251.  Move it up in the directory hierarchy to reflect this.

Additionally, make it truly optional.  At the moment, disabling
platform data while wl1251_sdio or wlcore_sdio are enabled doesn't
work, but it will be necessary when device tree support is
implemented.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>