arm: riscv: Remove additional ifdef from code guarded by CONFIG_IS_ENABLED

CONFIG_OF_LIBFDT is used twice for guarding the same code. It is enough to
do it once that's why remove additional ifdefs from arm and risc-v code.

Fixes: 0c303f9a ("image: Drop IMAGE_ENABLE_OF_LIBFDT")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Link: https://lore.kernel.org/r/f8e3ff9124195cbd957874de9a65ef79760ef5e7.1657183634.git.michal.simek@amd.com