ARM: OMAP2+: Fix build if CONFIG_SMP is not set

Looks like I only partially fixed up things if CONFIG_SMP
is not set for the recent kexec changes. We don't have
boot_secondary available without SMP as reported by Arnd.

Fixes: 0573b957fc21 ("ARM: OMAP4+: Prevent CPU1 related hang with kexec")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>