ARM: shmobile: rcar-gen2: Stop compiling headsmp-apmu on !SMP

As of commit cad160ed ("ARM: shmobile: Convert file to use
cntvoff"), there's no non-SMP code left in headsmp-apmu.S.

Hence build the file for SMP only, and drop the no longer needed check
for CONFIG_SMP inside the file.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>