soc: ti: k3-ringacc: Move state tracking variables under a struct

Move the free, occ, windex and rindex under a struct. We can use memset to
zero them and it will allow a cleaner way to extend driver functionality in
the future,

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>