smcinvoke: Rename size_add to smci_size_add

Rename size_add api of smcivoke driver to smci_size_add, as
smcinvoke's size_add is giving redefinition error with the size_add
api recently added in upstream patch 'overflow: Implement size_t
saturating arithmetic helpers'.
cherry-pick hash 4dab23059f51e324451b8fe02debc3df8812afd8.

Change-Id: I864c3065e63f53a3f4473edc24dc96d067dbfe60
Signed-off-by: Divisha Bisht <quic_divibish@quicinc.com>