serial: omap: Initialize platform_data

Otherwise serial driver would crash accessing platform_data that was
not initialized in functions like:

        serial_omap_pm(...)
        ...
        if (!state && pdata->enable_wakeup)
                   ^^^^^^^
        ...

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Alexander Savchenko <oleksandr.savchenko@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>