MIPS: ZBOOT: Define program header for text loadable segment

There is currently no corresponding ELF program header for the "text"
loadable segment which is confusing for some bootloader out there such
as CFE because it expects to find a program header matching the segment
it is trying to load. The Linux kernel ELF binary "vmlinux" has a
similar program header for the text segment so we just mimic this here
too.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: blogic@openwrt.org
Cc: james.hogan@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/5827/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>