sparc64: expand LDC interface

Add the following LDC APIs which are planned to be used by
LDC clients in the future:
- ldc_set_state: Sets given LDC channel to given state
- ldc_mode: Returns the mode of given LDC channel
- ldc_print: Prints info about given LDC channel
- ldc_rx_reset: Reset the RX queue of given LDC channel

Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Aaron Young <aaron.young@oracle.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>