In preparation to support multi-platform kernels, move all the dtb targets
out of the mach Makefile.boot and into the arch/arm/boot/dts/Makefile
which is closer to the sources.

DTBs are only built when CONFIG_OF is enabled and now use top level
CONFIG_ARCH_xxx instead of chip or board specific config options.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Viresh Kumar <viresh.linux@gmail.com>
Cc: Shiraz Hashim <shiraz.hashim@st.com>
Cc: Rajeev Kumar <rajeev-dlh.kumar@st.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Cc: Linus Walleij <linus.walleij@linaro.org>