phy: phy-brcm-sata: Allow configuration SATA AFE TX amplitude

Read the 'brcm,tx-amplitude-millivolt' property from Device Tree and
propagate its value into the appropriate test transmit register to
change the TX amplitude.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201022205056.233879-3-f.fainelli@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>