ARM: shmobile: fix platsmp.c build when ARCH_SH73A0=n

Fix build error in the case of SMP=y but ARCH_SH73A0=n
introduced by:

9601e873 ARM: shmobile: fix smp build

The use of of_machine_is_compatible() will link in the
the SoC-specific symbols:
"sh73a0_get_core_count", "sh73a0_smp_prepare_cpus",
"sh73a0_secondary_init" and "sh73a0_boot_secondary".

This patch adds an ugly #ifdef wrapper as a stop-gap
solution.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>