Set PHY rxc-skew-ps to 1500 and all other values to their default values.

This is intended to to address failures in the case of 1Gbps communication
using the salvator-x board with the KSZ9031RNX phy. This has been
reported to occur with both the r8a7795 (H3) and r8a7796 (M3-W) SoCs.

Based in a similar patch for the r8a7796 salvator-x by Kazuya Mizuguchi.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>