ARM: sunxi: enable big-endian

With previous fixes to stmmac, sun4i-emac and sun4i-mmc at least the
Cubieboard2 incarnation of sunxi works correctly in big-endian mode.
Allow the selection of big-endian.

Tested using a Cubieboard2 DualCard and the following
hardware-components:
- GMAC (stmmac)
- EMAC (sun4i-emac)
- SATA
- SPI
- UART
- security system
- touchscreen controller hwmon functionality
- pinctrl (custom interrupt for SPI ethernet controller)
- watchdog as reset controller
- onboard RTC
- DS1307 I2C RTC
- sound codec and with this indirectly the DMA engine

Signed-off-by: Michael Weiser <michael.weiser@gmx.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>