ARM: 6962/1: mach-h720x: fix build error

The h7201/h7202 machines did not build since they define
ARM_DMA_ZONE_OFFSET but do not select ZONE_DMA. Fix it up by
selecting ZONE_DMA in their Kconfig.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>