ARM: dts: fix typo in makefile for alpine-db

File uses dash in the filename, not underscore.
Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/boot/dts/Makefile

 ifeq ($(CONFIG_OF),y)
 
 dtb-$(CONFIG_ARCH_ALPINE) += \
-	alpine_db.dtb
+	alpine-db.dtb
 dtb-$(CONFIG_MACH_ASM9260) += \
 	alphascale-asm9260-devkit.dtb
 # Keep at91 dtb files sorted alphabetically for each SoC