lib: utils/fdt: Use byteorder conversion functions in libfdt_env.h

FDT follows big-endian and CPU can be little or big
endian as per the implementation.
libfdt_env.h defines function for conversion between
fdt and cpu byteorder according to the endianness.

Currently, libfdt_env.h defines custom byte swapping
macros and then undefines them. Instead, use the generic
endianness conversion functions

Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>