refactor(zynqmp): refactor custom sip service

Refactored the custom SIP service code and relocated it to a
common directory to support additional platforms and enhance
code readability. Renamed ZYNQMP_SIP_SVC_CUSTOM to a generic
SOC_SIP_SVC_CUSTOM for better with other across SoCs

Change-Id: Id114f717d0397e257a831c7d5b67104c2c766d09
Signed-off-by: Prasad Kummari <prasad.kummari@amd.com>
4 files changed