kbuild: Install dtb files as 0644 in Makefile.dtbinst

The compiled dtb files aren't executable, so install them with 0644 as their
permission mode, instead of defaulting to 0755 for the permission mode and
installing them with the executable bits set.

Some Linux distributions, including Debian, [1][2][3] already include fixes
in their kernel package build recipes to change the dtb file permissions to
0644 in their kernel packages. These changes, when additionally propagated
into the long-term kernel versions, will allow such distributions to remove
their downstream fixes.

[1] https://salsa.debian.org/kernel-team/linux/-/merge_requests/642
[2] https://salsa.debian.org/kernel-team/linux/-/merge_requests/749
[3] https://salsa.debian.org/kernel-team/linux/-/blob/debian/6.8.12-1/debian/rules.real#L193

Cc: Diederik de Haas <didi.debian@cknow.org>
Cc: <stable@vger.kernel.org>
Fixes: aefd80307a05 ("kbuild: refactor Makefile.dtbinst more")
Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>