ARM: 6763/1: pl011: add optional RX DMA to PL011 v2

This adds an optional RX DMA codepath for the devices that
support this by using the apropriate burst sizes instead of
pulling single bytes.

Includes portions of code written by Russell King during
a PL08x hacking session.

This has been tested on U300 and Ux500.
Tested-by: Jerzy Kasenberg <jerzy.kasenberg@tieto.com>
Tested-by: Grzegorz Sygieda <grzegorz.sygieda@tieto.com>
Tested-by: Marcin Mielczarczyk <marcin.mielczarczyk@tieto.com>
Signed-off-by: Per Forlin <per.friden@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>