linker: add bits for pinned regions

This adds the necessary bits for linker scripts and source code
to specify which symbols need to be pinned in memory. This is
needed for demand paging as some functions and data must reside
in memory all the time and cannot be paged out (e.g. paging,
scheduler, and interrupt routines for functionality).

This is up to the arch/SoC/board to define the sections in
their linker scripts as the pinned section may need special
alignment which cannot be done in common script snippets.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>