hikey960: platform.mk: Remove FIP_ADD_IMG SCP_BL2

hikey960: platform.mk: Remove FIP_ADD_IMG SCP_BL2

The line
$(eval $(call FIP_ADD_IMG,SCP_BL2,--scp-fw))
is removed from
plat/hisilicon/hikey960/platform.mk
to clear the warning below:

Makefile:544: warning: overriding commands for target `check_SCP_BL2'
plat/hisilicon/hikey960/platform.mk:13: warning: ignoring old commands for
target `check_SCP_BL2'

$(eval $(call FIP_ADD_IMG,SCP_BL2,--scp-fw))
already exists in
Makefile
and applies to plat hikey960 so is redundant in
plat/hisilicon/hikey960/platform.mk

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>