nfp: add nfp_cpp_area_size() accessor

Allow users outside of core reading area sizes.  This was not needed
previously because whatever entity created the area would usually know
what size it asked for.  The nfp_rtsym_map() helper, however, will
allocate the area based on the size of an RT-symbol with given name.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>