net: phy: add Motorcomm YT8531S phy id.

We added patch for motorcomm.c to support YT8531S. This patch has
been tested on AM335x platform which has one YT8531S interface
card and passed all test cases.
The tested cases indluding: YT8531S UTP function with support of
10M/100M/1000M; YT8531S Fiber function with support of 100M/1000M;
and YT8531S Combo function that supports auto detection of media type.

Since most functions of YT8531S are similar to YT8521 and we reuse some
codes for YT8521 in the patch file.

Signed-off-by: Frank <Frank.Sae@motor-comm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>