include: types: Add typedefs for endianness

If any variable/memory-location follows certain
endianness then its important to annotate it properly
so that proper conversion can be done before read/write
from that variable/memory.

Also, use these new typedefs in libfdt_env.h for deriving
its own custom fdtX_t types

Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>