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

docs/plat/marvell/armada/build.rst

@@ -174,6 +174,10 @@ There are several build options:
 
             - SATA - SATA device boot
 
+                Image needs to be stored at disk LBA 0 or at disk partition with
+                MBR type 0x4d (ASCII 'M' as in Marvell) or at disk partition with
+                GPT name ``MARVELL BOOT PARTITION``.
+
 - PARTNUM
 
         For Armada37x0 only, the boot partition number, default is 0.