lrdp-v2: add container_of and sibling_field_addr convenience functions

Some common operations include getting the parent structure of some
embedded field as well as getting "sibling" fields within the parent
structure. Add some convenience methods to the gdbmi module (as well as
wrappers in the RamDump class) for this.

Change-Id: Ic65d7e60b930af8c73384a30b6b0dd84fb55f09c