mmc: dove: fix missing MACH_DOVE dependency

DT-enabled Dove moved over from ARCH_DOVE in mach-dove to MACH_DOVE in
mach-mvebu. As non-DT ARCH_DOVE will stay to rot for a while, add a new
DT-only MACH_DOVE Kconfig. This slipped through the cracks and now is
a fix to allow to build Dove's SDHCI driver for mach-mvebu on v3.15-rc.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>