ata: ahci: mvebu: remove stale comment

For Armada-38x (32-bit) SoCs, PM platform support has been added since:
commit 32f9494c ("ARM: mvebu: prepare pm-board.c for the
                      introduction of Armada 38x support")
commit 3cbd6a6c ("ARM: mvebu: Add standby support")

For Armada 64-bit SoCs, like the A3700 also using this AHCI driver, PM
platform support has always existed.

There are even suspend/resume hooks in this driver since:
commit d6ecf158 ("ata: ahci_mvebu: add suspend/resume support")

Remove the stale comment at the end of this driver stating that all
the above does not exist yet.

Fixes: d6ecf158

 ("ata: ahci_mvebu: add suspend/resume support")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>