ARM: davinci: limit DT support to DA850

When da8xx-dt.c is built with onlu DA830 support but not DA850
support enabled, we get a compiler warning about unused symbols:

arch/arm/mach-davinci/da8xx-dt.c:28:20: warning: 'da8xx_init_irq' defined but not used [-Wunused-function]
 static void __init da8xx_init_irq(void)
arch/arm/mach-davinci/da8xx-dt.c:33:30: warning: 'da850_auxdata_lookup' defined but not used [-Wunused-variable]
 static struct of_dev_auxdata da850_auxdata_lookup[] __initdata = {

Obviously none of the file make sense for DA830, so we should not
even attempt this, so we can avoid the warning by ensuring it is
only built for 850, not 830.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>