ssb: register serial flash as platform device

This allows writing MTD driver working as a platform driver. In
platform_data it will receive struct ssb_sflash, which contains all
important data about flash (window, size).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>