vmm_obj_service: Handle mapping of physical memory.

Updates share_buffer to handle physical memory outside of Trusty's
default mapped address space. Additionally sets the size of the source
data in the first usize of the buffer since full pages must be mapped.

Bug: 359345227
Test: Map desktop boot params into gsc_service TA.
Change-Id: I2dcd5fc1d114ab783e5f74e547731ef55c9c86bf