net: stmmac: add new MAC method set_lpi_mode()

net: stmmac: add new MAC method set_lpi_mode()

Add a new method to control LPI mode configuration. This is architected
to have three configuration states: LPI disabled, LPI forced (active),
or LPI under hardware timer control. This reflects the three modes
which the main body of the driver wishes to deal with.

We pass in whether transmit clock gating should be used, and the
hardware timer value in microseconds to be set when using hardware
timer control.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tffds-003ZIT-E8@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>