SPEAr6xx: Add compilation support for dtbs using 'make dtbs'

'make dtbs' compiles all dtbs for a family. For this we need to add entries for
all board dtb's in Makefile.boot file. This patch does it.
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>

arch/arm/mach-spear6xx/Makefile.boot

 zreladdr-y	+= 0x00008000
 params_phys-y	:= 0x00000100
 initrd_phys-y	:= 0x00800000
+
+dtb-$(CONFIG_BOARD_SPEAR600_DT)	+= spear600-evb.dtb