ARM: 6462/1: EP93xx: Document DMA M2P API

Add kernel-doc documentation for the EP93xx DMA memory to
peripheral/peripheral to memory API.

Signed-off-by: Ryan Mallon <ryan@bluewatersys.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>