arm: mvebu: a38x: serdes: Add comments for hws_pex_config() code

Add comments to understand what this magic code is doing.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>