docs: marvell: Update info about BOOTDEV=SATA

Information is taken from the post https://lists.denx.de/pipermail/u-boot/2017-July/299351.html

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I5f608e135ec56685a3e2b986a52670540d48a4bf
1 file changed