fix(corstone-1000): add cpu_helpers.S to platform.mk

Add Platform related dependency in Makefile

Reviewed-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Signed-off-by: Gauri Sahnan <Gauri.Sahnan@arm.com>
Change-Id: Idecb84233d3e0c386bf0b7f6d57cbebd38875f28
1 file changed