ARM: realview: fix device tree build

As it happens, two obj-$(CONFIG_FOO) += foo.o variables are
above obj-y := core.o, which doesn't work: the += directives
need to add something to the build and doesn't work if obj-y
is not set first, so move the obj-y to be on top and everything
builds nicely again.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>