ARM: kirkwood: add missing kexec.h include

Fixes the following build error when CONFIG_KEXEC is enabled:
  CC      arch/arm/mach-kirkwood/board-dt.o
arch/arm/mach-kirkwood/board-dt.c: In function 'kirkwood_dt_init':
arch/arm/mach-kirkwood/board-dt.c:52:2: error: 'kexec_reinit' undeclared (first use in this function)
arch/arm/mach-kirkwood/board-dt.c:52:2: note: each undeclared identifier is reported only once for each function it appears in

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
[v4, rebase onto recent Linus for repost]
[v3, speak actual English in the commit message, thanks Sergei Shtylyov]
[v2, using linux/kexec.h not asm/kexec.h]
Signed-off-by: Jason Cooper <jason@lakedaemon.net>