dt-binding: ptp_qoriq: add little-endian support

Specify "little-endian" property if the 1588 timer IP block
is little-endian mode. The default endian mode is big-endian.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>