feat(zynqmp): add support for custom sip service

feat(zynqmp): add support for custom sip service

Add support for custom sip service.
Bare minimum implementation for custom_smc_handler is provided
by platform. Actual definition for custom_smc_handler will be provided
by custom pkg.

This feature is going to be used by external libraries. For example
for checking it's status.

The similar approach is also used by qti/{sc7180,sc7280} platforms
by providing a way to select QTISECLIB_PATH.

This code is providing a generic way how to wire any code
via custom $(CUSTOM_PKG_PATH)/custom_pkg.mk makefile with also an
option to wire custom SMC. SMC functionality depends on "package".

Change-Id: Icedffd582f76f89fc399b0bb2e05cdaee9b743a0
Signed-off-by: Amit Nagal <amit.nagal@amd.com>