rockchip: make_fit_atf: ignore empty PT_LOAD segment

The linker sometimes creates PT_LOAD segments with length (p_filesz) zero
as described in https://man7.org/linux/man-pages/man5/elf.5.html

. This
leads to build failures. We should ignore empty segments.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>