ata: ahci: mvebu: add Armada 3700 initialization needed for S2RAM

A3700 comphy initialization is done in the firmware (TF-A). Looking at
the SATA PHY initialization routine, there is a comment about "vendor
specific" registers. Two registers are mentioned. They are not
initialized there in the firmware because they are AHCI related, while
the firmware at this location does only PHY configuration. The
solution to avoid doing such initialization is relying on U-Boot.

While this work at boot time, U-Boot is definitely not going to run
during a resume after suspending to RAM.

Two possible solutions were considered:
* Fixing the firmware.
* Fixing the kernel driver.

The first solution would take ages to propagate, while the second
solution is easy to implement as the driver as been a little bit
reworked to prepare for such platform configuration. Hence, this patch
adds an Armada 3700 configuration function to set these two registers
both at boot time (in the probe) and after a suspend (in the resume
path).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>