wl12xx: add a platform device to the spi module

The platform device will be used to match the platform driver that
will be implemented by the core module.

Signed-off-by: Felipe Balbi <balbi@ti.com>
[forward-ported, cleaned-up and rephrased commit message]
[call platform_device_add() instead of platform_device_register()]
[store alloc'ed device platform directly in glue->core]
[fixed the length of memset(res...)]
Signed-off-by: Luciano Coelho <coelho@ti.com>