mlxsw: Add ability to control transceiver modules' power mode

Implement support for ethtool_ops::.get_module_power_mode and
ethtool_ops::set_module_power_mode.

The get operation is implemented using the Management Cable IO and
Notifications (MCION) register that reports the operational power mode
of the module and its presence. In case a module is not present, its
operational power mode is not reported to ethtool and user space. If not
set before, the power mode policy is reported as "high", which is the
default on Mellanox systems.

The set operation is implemented using the Port Module Memory Map
Properties (PMMP) register. The register instructs the device's firmware
to transition a plugged-in module to / out of low power mode by writing
to its memory map.

When the power mode policy is set to 'auto', a module will not
transition to low power mode as long as any ports using it are
administratively up. Example:

 # devlink port split swp11 count 4

 # ethtool --set-module swp11s0 power-mode-policy auto

 $ ethtool --show-module swp11s0
 Module parameters for swp11s0:
 power-mode-policy auto
 power-mode low

 # ip link set dev swp11s0 up

 # ip link set dev swp11s1 up

 $ ethtool --show-module swp11s0
 Module parameters for swp11s0:
 power-mode-policy auto
 power-mode high

 # ip link set dev swp11s1 down

 $ ethtool --show-module swp11s0
 Module parameters for swp11s0:
 power-mode-policy auto
 power-mode high

 # ip link set dev swp11s0 down

 $ ethtool --show-module swp11s0
 Module parameters for swp11s0:
 power-mode-policy auto
 power-mode low

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>