arm64: remove unnecessary prom.h include

Remove unnecessary prom.h include in preparation to make prom.h optional.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/init.c

@@ -31,7 +31,6 @@
 #include <linux/sort.h>
 #include <linux/of_fdt.h>
 
-#include <asm/prom.h>
 #include <asm/sections.h>
 #include <asm/setup.h>
 #include <asm/sizes.h>