arm: mvebu: a38x: serdes: Add comments and use macros in PCIe code

arm: mvebu: a38x: serdes: Add comments and use macros in PCIe code

Replace magic register offsets by macros to make code more readable.
Add comments about what this 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>