wl12xx: add platform driver to the core module

Nnow that we have a platform_device on both glue layers, add a
platform_driver to the core driver.

It's currently an empty platform_driver but more functionality will be
added on later patches.

Signed-off-by: Felipe Balbi <balbi@ti.com>
[forward-ported, cleaned-up and rephrased commit message]
[added platform_driver.driver initialization]
Signed-off-by: Luciano Coelho <coelho@ti.com>