ARM: move all dtb targets out of Makefile.boot

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>

arch/arm/mach-imx/Makefile.boot

@@ -37,10 +37,3 @@ initrd_phys-$(CONFIG_SOC_IMX53)	:= 0x70800000
 zreladdr-$(CONFIG_SOC_IMX6Q)	+= 0x10008000
 params_phys-$(CONFIG_SOC_IMX6Q)	:= 0x10000100
 initrd_phys-$(CONFIG_SOC_IMX6Q)	:= 0x10800000
-
-dtb-$(CONFIG_MACH_IMX51_DT) += imx51-babbage.dtb
-dtb-$(CONFIG_MACH_IMX53_DT) += imx53-ard.dtb imx53-evk.dtb \
-			       imx53-qsb.dtb imx53-smd.dtb
-dtb-$(CONFIG_SOC_IMX6Q)	+= imx6q-arm2.dtb \
-			   imx6q-sabrelite.dtb \
-			   imx6q-sabresd.dtb \