Add support for RTC mode if it is supported by firmware.  In RTC
mode, the PHC is set to the 64-bit clock.  Because the legacy interface
is 48-bit, the driver still has to keep track of the upper 16 bits and
handle the rollover.

Cc: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>