ARMv8: Allow SiP service extensions on top of PSCI code

ARMv8: Allow SiP service extensions on top of PSCI code

Allow PSCI layer to handle any SiP service functions added by
platform vendors. PSCI layer will look for SiP service function
in the SiP function table located in '._secure_svc_tbl_entries'
section if the SMC function identifier is not found in the PSCI
standard functions table. Use DECLARE_SECURE_SVC macro to declare
and add platform specific SiP service function.
This new section '._secure_svc_tbl_entries' is located next to
'._secure.text' section. Refer to arch/arm/cpu/armv8/u-boot.lds.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>