For builtin dtbs all compiled dtbs matching section .dtb.rodata.*
are collected in a single section. Since every dtb is compiled as
uncompressed and also as compressed binary each dtb ends up twice
in the section. Let's put the compressed variants in .dtbz.rodata.*
sections rather than .dtb.rodata.*.z so they end up in the binary
only once.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Daniel Krüger <daniel.krueger@systec-electronic.com>