phy: marvell: add RX training command

phy: marvell: add RX training command

This patch adds support for running RX training using new command called
"rx_training"
Usage:
rx_training - rx_training <cp id> <comphy id>

RX training allows to improve link quality (for SFI mode)
by running training sequence between us and the link partner,
this allows to reach better link quality then using static configuration.

Change-Id: I818fe67ccaf19a87af50d4c34a9db7d6802049a5
Signed-off-by: Igal Liberman <igall@marvell.com>
Signed-off-by: Marcin Wojtas <mw@semihalf.com>