ARM: OMAP2+: Fix warning for hwspinlock omap_postcore_initcall

Commit 816a65ef4 (ARM: OMAP2+: Limit omap initcalls to omap only on
multiplatform kernels) fixed up things for multiplatform booting
but failed to include soc.h causing a new warning:

arch/arm/mach-omap2/hwspinlock.c:60:1: warning: data definition has no type or storage class
arch/arm/mach-omap2/hwspinlock.c:60:1: warning: type defaults to 'int' in declaration of 'omap_postcore_initcall'
arch/arm/mach-omap2/hwspinlock.c:60:1: warning: parameter names (without types) in function declaration
arch/arm/mach-omap2/hwspinlock.c:31:122: warning: 'hwspinlocks_init' defined but not used

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>