json: make it 64-bit compatible

The struct json_obj_descr definition allocates only 2 bits for type
alignment. Instead of using them literally minus 1 to encode 1, 2, or 4,
let's store the alignment's shift value instead so that 1, 2, 4 or 8 can
be encoded with the same 2 bits to accommodate 64-bit builds.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>