ARM: versatile: fix missing <plat/platsmp.h> include

Fix two missing function declarations by including the
<plat/platsmp.h> file where they are defined. Fixes:

arch/arm/plat-versatile/platsmp.c:35:6: warning: symbol 'versatile_secondary_init' was not declared. Should it be static?
arch/arm/plat-versatile/platsmp.c:50:5: warning: symbol 'versatile_boot_secondary' was not declared. Should it be static?

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>