RxFifoSz, TxFifoSz, RxFifoAddr, TxFifoAddr
are all indexed registers.

So before doing a context save or restore, INDEX register
should be set, then only one gets to the right register offset.
Signed-off-by: NVikram Pandita <vikram.pandita@ti.com>
Signed-off-by: NAnand Gadiyar <gadiyar@ti.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>