Staging: ti-st: remove st_get_plat_device

In order to support multiple ST platform devices, a new symbol
'st_get_plat_device' earlier needed to be exported by the arch/XX/brd-XX.c
file which intends to add the ST platform device.

On removing this dependency, now inside ST driver maintain the array of
ST platform devices that would be registered.
As of now let id=0, as and when we end up having such platforms
where mutliple ST devices can exist, id would come from
protocol drivers (BT, FM and GPS) as to on which platform device
they want to register to.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Anca Emanuel <anca.emanuel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>