ARM: shmobile: rcar-gen2: fix non-SMP build

A bugfix for the SMP case broke the build for the UP case:

arch/arm/mach-shmobile/headsmp-apmu.o: In function `shmobile_boot_apmu':
(.text+0x34): undefined reference to `secondary_startup'

The assembler file mixes code that is used for SMP with code
that we also need on a single-CPU build, so I'm leaving it
always enabled in the Makefile, but enclose the SMP code
in an #ifdef.

Fixes: fd45a136ff6 ("ARM: shmobile: rcar-gen2: Make sure CNTVOFF is initialized on CA7/15")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>