docs(a3k): fix information about SATA GPT booting

Armada 3720 BootROM searches for GPT partition with partition type GUID
6828311A-BA55-42A4-BCDE-A89BB5EDECAE and completely ignores GPT
partition name. It does not check for "MARVELL BOOT PARTITION".

This fact is incorrectly documented even in official Marvell Armada 3700
Functional Specification.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I35279f39de2d12148fc16f2730a9a074dc0b58eb