ARM: mach-shmobile: headsmp.S build fix

Git commit f4117ac9
introduced PLAT_PHYS_OFFSET, but headsmp.S was left
unchanged which results in a compile error:

  AS      arch/arm/mach-shmobile/headsmp.o
arch/arm/mach-shmobile/headsmp.S: Assembler messages:
arch/arm/mach-shmobile/headsmp.S:27: Error: undefined symbol `secondary_startup' in operation
arch/arm/mach-shmobile/headsmp.S:27: Error: undefined symbol `PHYS_OFFSET' in operation
make[1]: *** [arch/arm/mach-shmobile/headsmp.o] Error 1
make[1]: *** Waiting for unfinished jobs....

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>